Vanity.

I've been drawing self portraits since I was an art student, partly because it's good drawing practice, but mainly because it's easier than persuading someone to sit for me.

Drawing the human face or figure means that any mistakes leap out at you, so you're less inclined to let your drawing become lax. Drawing yourself means you've always got a model, but it also means you get to examine your face somewhat more closely than you usually would, and see the harm that time is doing. (As well as the flaws that came built in - little lapses in symmetry, features that are too big or too small, a nose that points off to the side.)

If you have any vanity, drawing your face pretty much takes it away. Having said that, when I look back at photographs of me taken years ago, I wonder how that fresh faced innocent ever survived to become the evil old monkey into which I am slowly transforming. Altogether, I think I prefer the look of now me.