It is generally assumed that happiness is not only the abstract "aim of our existence," but also one of the main sources of motivation in people's decision making. People choose to do one thing or the other based on whether they think it will make them happy or not. Research has shown that we're not always that good at predicting what will make us happy.
My observation both of myself and others has led me to the conclusion that, while happiness is indeed important to people, they generally assume (often incorrectly) that it will follow as a by-product of other, more important pursuits, which can be summed up as "biological success."
Biological success is a complex concept encompassing physical health, sexual attractiveness, social standing (status, recognition, influence), material well-being, security, and prospects for the future. These are the kinds of things that probably most affect the likelihood that one will pass on one's genes to future generations. What evolution prescribes is an often conflicted mixture of things that "feel right" on a personal level and things that will gain the approval of one's particular reference groups and provide one's offspring with better chances of enjoying a secure position in that society.
According to this evolutionary logic, it seems that as soon as "sufficient" success is achieved in one area, one's attention shifts to another area of greater perceived weakness. This might be one way of looking at why people gain weight after marriage, sacrifice a degree of physical fitness to own a car (especially in poorer countries), or give up hobbies and social lives to become workaholics. In many contexts, these are choices that increase biological success — as long as most of the people around them are making the same choices. The logic is similar to that of an arms race.
There is also a minority subculture of people who seem to do the opposite — give up things that would win them broad social approval in order to maximize physical fitness, richness in their emotional lives, enjoyment from hobbies and non-monetary pursuits, etc. I sympathize with this subculture, but I think they enjoy, on average, less reproductive success (I don't have data). Even these people are highly subject to social influence; they simply band into communities that have a somewhat different value system, and proceed to do things that will earn them esteem within that community.
At any rate, it seems exceedingly rare to meet someone outside of the psychotherapist's office for whom maximizing happiness is the chief aim of his/her existence. Even for these people, I think unhappiness only really becomes a problem when it is seen as a cause of failure in other areas of life.
My own experience in studying happiness has been eye-opening. When I began my practice of self-quantifying (tracking various aspects of my everyday life through an increasingly sophisticated rating system), it was to learn more about how I was living my physical life. It became clear to me that my emotional life had an effect on my physical life, so I began to track that as well. Then, I added components of my intellectual life. I figured I needed a composite score, so I created an algorithm that accurately reflected my feeling of well-being for that day based on all the separate aspects. I also commented on specific things that influenced my well-being over that day.
While this exercise was initially begun to get control over my physical health and was later expanded as an engaging intellectual exercise, it has served to help me dedicate a bit more of my attention to maximizing my own happiness as opposed to pursuing other goals. In my case, this is a much-needed shift. Now I can see how my quality of life has slowly been improving as a result. (Or at least so it seems at the moment. Like chronic dieters, maybe I'll just abandon the entire process when something really bad happens, then resume tracking when the worst is already behind me.) The act of consciously observing something changes behavior. At the same time, I note that I consistently do things, or allow things to happen, that hurt my feeling of happiness.
For instance, virtually every time I make a move between countries, I have a depressed sense of happiness for at least several days before and after the move due to the stress of finishing up last-minute business, packing, losing sleep, being packed together with unfamiliar people for many hours, and figuring out my life in the new location. Yet I continue to do this time after time, believing that the stress is worth it because of some other more important objective.
As I become more and more aware of what exactly is affecting my level of happiness, I increasingly gain the ability to make choices about whether or not to submit myself to different things whose effects on my happiness are now well established. I also spend more time thinking about whether there is some way to "tweak" things so that the same stimulus won't have a negative effect. And I've begun to place more value on things that have clearly been shown to enhance happiness, treating them like food or medication that needs to be taken in certain amounts on a regular basis.
Now, if I can only get other people to start taking their own happiness more seriously, then that will raise the prestige of this activity and I can use my expertise in pursuing happiness to achieve greater biological success...