When I see something especially beautiful or otherwise captivating, I want to own it, devour it, and possess it completely. Sometimes I can just buy the object in question, if it's a physical object, and sometimes it's enough to save the file, if it's a digital image, and then I can look at those things whenever I want. But if I sit down and sketch them, then it's a more personal, more engaging, and more complete experience of their beauty than simply looking at them. This wish to own beauty is the first thing that compels me to draw. All other reasons I have for drawing came about only after I started drawing.
Also, drawing my hands or other people in the room is great if I'm in a boring class or meeting.