My immediate answer was "because I want to." I draw because I simply like to draw. However, the more I thought about it the more I decided that it wasn't because I "want" to but because I "need" to. Drawing or doing artistic things in general keeps me sane. It is stress relief and relief from boredom, and it lets me express myself. BUT as I thought about this even more - mainly thinking about what i choose to draw and what drawings inspire me - I realized that it's a little bit more than that. Most of the pieces that are inspirational to me are portraits or drawings of people in general. I draw because I love people... It's the same reason that I love psychology! I am obsessed with the mind and why we do things or act a certain way. I love learning about different emotions and the psychology behind each one just as much as I love learning about how children's brains develop. I am passionate about the mind, and it is why I love drawing portraits and people in subways... I am entranced by both the beauty of the mind, and the beauty of the way it is expressed in a person's face. Or at least, that is what I've decided for the time being.
tl;dr: I do what I do in terms of art both because I need to, and because I am obsessed with people.