Why do you do what you do?

So I was working on my newest project this week and was focusing on the motivation behind the characters. See my main character has two motivations, to keep the girl safe, and also to find out who is behind this mess.

My antagonist have the single motivation of power, by means of the girl.

Now the fun part in this is that it sets itself up perfectly for conflict. The antagonist wants what the hero has, so they clash just on their driving forces. Now this leaves the girl as a sort of rope in a big battle of tug of war, so I had to really want to get down to what her motivation was. Staying alive is real enough, but not very original, so I added in the desire to find the truth and begin to trust someone fully, because that's what I think is a better motivator.

So when you sit down and write, do you know your characters motivations and goals from the start. I have to admit for the above, I knew my hero's and antagonists, but the story wasn't full until I realized that the girl had some too. So it's fleshed out the story, made it three dimensional so to speak.

What are some of your favorite character motivations?