The jobs that no one likes to do are done by machines. Haven't you read your science fiction?

I think that being taught that working is the right thing to do would be enough to keep most people doing it. Society has a lot of power over the individual. Also, it seems to be a recurring theme in star trek that these people are explorers--they're not producing much of anything, they're just keeping themselves occupied. Notice that the only 'work' they do is fighting wars and avoiding disaster. I'd like to think that humans naturally try to avoid disaster, and I know for a fact that they naturally fight wars.

Other jobs seen in Star Trek include artists, traders, therapists, doctors, mechanics, and holosuite programmers. Plenty to do.

And there's always something to buy--a vacation on Risa, strange alien art, and holoprogrames. And don't forget the joy of hoarding piles of gold-pressed latinum.