I don't always agree with Mark Cuban, but I think he makes a good point on this when he talks about the folly in the adage, "follow your passions." His point is that you should follow your efforts as that leads you to be better at something, which in turn grows your passions in it. You can read his words for yourself: http://blogmaverick.com/2012/03/18/dont-follow-your-passion-....
Even if you do not have any 'passion' for a field, I think it is important to have some kind of strong intrinsic motivation; to help people, to further Humanity's knowledge, or whatever else. Practice for the sake of practice is incredibly hard to maintain.
