Sometimes we know we have to do something. It may be hard to pinpoint why, but we've got this feeling that we've just got to do it.
It may be something as mundane as deciding what to eat for dinner, to something life-changing like deciding that medical school is not for you in your senior year.
Chris Guillebeau talks about this in his post, 'The Individual as Hero', that "almost everyone who pursues a quest has a hard time explaining their reasons". He claims:
The compulsion to pursue a dream is accompanied by its own sustaining power.
Interestingly, the bigger the ambition, the more complex the dream, the harder it is to hold all those possibilities in your conscious mind. And yet, the more inexplicable that urge, the more alluring the attraction. Numinous.
I agree with Chris, we'd all be better off if we'd set our mind onto a dream, even if we don't know why.