Humans are hardwired to exhibit some behaviors; this is called instinct. Today, instincts like sexual desire and hunger are beneficial. However, for a transhuman who can optimize his behaviour better than natural evolution could, those instincts would be limiting.
If they choose to have conscious and voluntary control and selection over their instincts, love, sex and food may not mean as much to transhumans as they do to homo sapiens. Trans or posthumans could choose to ignore, manipulate and turn off the instincts of homo sapiens, or they could decide that they want to keep some of them.
A transhuman person or body could be able to track the effects drugs and stimulation has on the brain, and one could choose to manipulate such effects more exactly. For example, one could simulate what humans experience as love or one could choose to make the emotion or experience redundant to the posthuman body.
Of course, natural selection has shaped and continues to shape the drives and instincts humans have as a species. While they may not be optimal in a given contemporary environment, they have at least proven effective in producing human behavior that has been selected for in the environments humans have had to deal with in their recent evolutionary past.
Because of late, the environment humans live in has changed, it is not surprising that the innate drives humans find themselves with are not always optimally tuned to produce the most effective behavior. This is very likely to be the case in the future as well. There is always change requiring adaptation.
Rather than live in an environment a human is not suited for, the temptation is to use drugs or other means to alter the innate drives of the human so as to achieve certain ends.
The big question is: 'Whose Ends?'
A drug abuser may start their habit to escape some stress. The well known story is that the addiction to the drug ends up damaging the user even more.
The ends actually served by the chemical alterations, are those of the sellers of the drug, and maybe others, but are generally damaging to the drug abuser. However, drugs of abuse ( with the notable exceptions of Tobacco, and Alcohol ) tend to be outlawed so it is prescription drugs that seem more likely to be used to alter human instincts most widely in the future. It is interesting to think about whose ends the alterations produced by current and future prescription drugs serve.
Prolonged depression for no identifiable reason that interferes with one's ability to function, has been identified as a disease, and that disease is treated with Antidepressant Drugs. There is no proof that 'Chemical Imbalances in the Brain' cause depression, it being at least as likely that depression itself causes the 'Chemical Imbalances in the Brain' associated with depression.
Antidepressants work by altering the brain's chemistry rather than by helping a mind to identify articulate the root cause of the patient's troubles with an eye toward designing ways to deal with them. Since often, the patient's behavior in the depressed state happens to be part of the cause of their depressed state, in many cases the treatment actually works without anyone having known exactly why. Likely, when treatment with a drug fails, the patient's brain has learnt to cope with the drug, and the patient's correct assessment of their own situation returns them to the depressed state. A succession of new drugs are tried that work briefly until the patient's brain learns how to cope with them sufficiently to be sober despite them, with their mind returning to the depressed state.
This isn't to disparage antidepressants. The behavior of the depressed can and does contribute to the depression itself in a self feeding cycle, and it may indeed be easier, and more effective to chemically alter the brain to allow someone's life to 'reset' on its own rather than to try and delve into probably repressed root causes which may turn out to be unimportant after the reset.
There are also the Attention Deficit Disorder type diseases which are also treated via medication. Some of these medications are similar to speed, which is known to be used by students to cram for tests. It is known that methamphetamine users will engage in repetitive 'boring' tasks that require attention while high on the drug such as beachcombing, metal-detecting, or searching fields for stone arrowheads. Students who use speed-like drugs as a study aid feel a sense of accomplishment while studying material that is to boring to focus on for them without drugs.
Likely the difficulties faced by those diagnosed with ADD and/or ADHD are the same difficulties we all face when asked to do something we find boring, only magnified.
The feeling of boredom, and the inability to focus on boring things is human. It prevents people from wasting their energy unproductively, and drives innovation. Is the A student who completes a hundred long division math problems by hand smarter than the A student who could have done one hundred long division problems by hand if they could have stood the sheer boredom without letting errors due to carelessness or inattention creep in, but instead found a way to cheat convincingly using a calculator? No. Most likely the innovator who cheated is the smarter one. Unable to feel the innate human drive of boredom, the innovator would not have been as driven to innovate.
Neither is the non-cheater who just flunks math because they didn't hand in the homework they couldn't make themselves do because it was too boring dumber than the students equipped with a less pronounced anti-boredom drive. The situation of school is artificial, only recently encountered by humans, and it is not one that everyone is genetically adapted for.
Of course there are things that are boring to some but interesting to others. One student, call him Jack, might hang his ego on whether he gets an A feeling a charge of pleasure by completing even pointless assignments, while another student, call her Jill, might see that the assignment won't teach her anything, but will suck up her time and energy better spent doing something else. Jill may want the A as much as the Jack, and may even understand the material better than him, but she will find it much more difficult to complete the assignment and actually get the A than Jack will. Which student gets admitted to the college of their choice? Usually, it's Jack, not Jill.
Jill, who couldn't make herself do the pointless assignment might actually find the subject interesting on it's own, and she may read ahead in the textbook to learn about the subject without it being required, whereas Jack may find the subject inherently boring, only being interested if there is a grade involved.
If Jill takes drugs to overcome the limitation of being too bored by the pointless hoops she must jump through to make it through them, and so enters the field of her interest, will she find the workplace structured to accommodate Jacks? Will they have to keep taking the drugs to stave off boredom enough to keep her job?
Where is Jill's natural niche? Is it in the field of her interest, doing the job Jack now does, better than he could? Is it to take drugs to be able to compete with a Jack? Or is her natural niche somewhere else? Should she let nature take it's course, or should she take matters into her own hands to survive?
If she takes drugs to survive, has she really come through entirely? What kind of survival does she have to look forward to?
In the future it may become possible to cope with quite bleak cog-like lifestyles by taking prescription drugs. Even if accepting such an existence is harmful to one's humanity, if those who live that way have monopolized the various niches in society, then the alternative is non-existence. Your place in the world may be better served by someone else if only they took drugs. If they take drugs and displace you, do you take drugs yourself in hopes of displacing someone else?
Maybe only very few will have the mean$ to enjoy the privilege of remaining fully human.
And drugs are only one possibility for altering humans. Surgically implanted electronics in the brain might someday perform the same functions. Genetic manipulation is possible.
All these things are currently done to domesticated animals and plants for the ends of people. Instead those with money and power needing to force other people to serve their ends by forcefully imposing alteration on them, people will be willing to alter themselves to meet the ends of those with the means of providing them with a living. 'Those with the money and power' may not even be human at all. They may be financial machines. They may be faceless corporations.
Animals and plants domesticated themselves voluntarily because it gave them a survival advantage.
People planted the seeds they gathered. No wonder the ones most easily gathered by people became the next generation of plants.
Hungry animals accepting the food offered by people came to live with people and be dependent on them.
Hungry (for food or even unnecessaries) people might accept the benefits offered by others and so become dependent on and domesticated by them. In a world where people compete for niches, the temptation to alter ones self even to one's own detriment to better compete with others who are doing so may be too compelling to resist.
It is also worth considering the myriad ordinary ways humans attempt to fool their innate drives into thinking they have been satisfied. Masturbation is one example. Television, the internet, hobbies, and other diversions. When it becomes economically possible to partake of many diversionary activities, population tends to decrease unless there is immigration.
Over time, susceptibility to this should be selected against, but human drives as they are now, are currently prone to being diverted toward activities that empirically lead to lower population levels. That is, the first world is on average a 'toxic' environment for humans despite the high standard of living.
What is it about human nature makes people able to fool themselves into thinking their innate drives are being satisfied? Would people be less human without it? We may find out.