I'm probably going to get a bunch of 'because of my alarm clock' answers (or different variations of that), but regardless:

Why do you get up in the morning? What keeps you going? What are you living for?

You can be as simple or as philosophical as possible.

For me, I'm sure a lot of it could be attributed to me just living for the sake of living, but the two main driving forces in my life are spending time with my very, very close friends, and the pipe dream that I could (after finishing my degree and making some money off of it) end up with a career that I want.