As a player, I like there to be some motivation other than kill things and take their stuff, but I'm happy to go along with whatever the DM cooks up. For instance, in Steve Creech's (Ghostwind's) game, my character has sworn to protect a young nobleman. Originally she was supposed to get him to the city where his betrothed was waiting for him, get him safely married and then back home, but, well, a comet hit the planet. That sort of changed everyone's plans, but my character still considers her oath to this kid's (now most likely dead) mother in effect.
In the other game I play in, my character and my husband's got arrested for scamming drunk rich customers in a roadhouse and we're now arena slaves with a number of other folks (the other PCs, obviously). Thus, we do what we're told until we can figure out a way to escape.
As a DM, I try to provide a rich and detailed setting, with lots of NPCs doing whatever it is they do, complete with their individual motivations, tales of woe and whatnot. I expect my players to pick one of the breadcrumb trails I leave scattered around and follow it. I don't allow PCs of evil alignment, so generally the desire to leave the world a better place is motivation enough, but to the extent I can, I try to work their background details in here and there. I also make sure the players know that there are consequences for everythng they do. Good deeds gain them the attention of powerful benefactors, and stupid or foul deeds gain them less savory attention. So far, that has always been enough.
I assume that we're all playing because we want to. So why try to be obstructive? I just don't get it.