There was actually a Harvard study that started 75 years ago that asked this same question to alumni, and stayed with them to see how their answers changed over time.
http://www.huffingtonpost.com/2013/08/11/how-this-harvard-psycholo_n_3727229.html
The summary of the study is this, relationships drive satisfaction in life more than any other factor (money, power, success). Which is something that while Barney and friends have been preaching for years, people just seem to forget after a long period of time.
As for me, I'm sort of further evidence of this study. I work as a CNA in a nursing home and I see first hand the end results of what a lot of power and money gets you in the end. Practically nothing. My nursing home is pretty upscale and the people who live there generally pay out the ass to do so (upwards of $10k a month). Clearly these people all did something right in their lives to be able to afford to live there. But the percentage of them that actually have anything to live for still is split in half. And it's not just because of their age or deteriorating health. There are really two groups of residents at these facilities, the ones with visitors, and the ones without. The ones with regular, consistent visitors (family, friends, clergy) that come by are almost always pleasant and are just as eager to joke around and laugh about things as you or I would. But about half of them have visitors rarely, or not at all. These are usually the ones that are combative, their situational depression is worse, and their health actively deteriorates faster.
The study I linked to above shows a similar trend. The Harvard graduates all went on to varying successful careers and lifestyles, but universally what was determined was that love really is all that matters. A body withers away, and wealth becomes irrelevant when you get older, and all you really care about is who is going to laugh at your stupid jokes, or watch a sad movie with you, or listen to what you have to say, even if it hurts to hear it.
I suppose I derive fulfillment out of being that person for my residents, and the people I love. When I'm at school studying, eating right, or training on the mat, sure I'm doing it for my own enjoyment and betterment, but I also do it because I take pride in being the friend or family member that can be relied on, and I actively take care of myself so I can be there when I'm needed.
Also I like getting laid and playing Pokemon. Gotta have an outlet :D