That's a good question, judging by the number and variety of replies.

I'd suggest that in a way, things go the other way around. Instead of your concluding you should do something causing you to do it, instead I think you are (already) aiming to do something, and that drives you to figure out what you should do. The urge to do causes figuring out what to do, rather than the figuring causing the doing.

But that's a little over-simplified, as discovered by people trying to program robots that interact with the world. Deciding what to do at any given moment is distinctly non-trivial.