I've gotten this question a few times and it never fails to almost offend me.
Sometimes, on twitter, I'll ask for suggestions or ask if my followers on there want to request I do something. These are usually for warm-up purposes and the drawings are very very rarely fully fleshed out drawings. Other times, if a friend asks me to draw something, I'll do it. For a FRIEND. As in someone I know, and talk to, and interact with regularly.
If I don't say "Hey, it's request time!" or you're a friend of mine, why the Hell would I want to take the time to do your request for no return?
Not to sound like a tortured artist douche bag, but my work takes time. It takes effort. Time and effort that I don't like just giving away, and that I think is incredibly rude of others to presume that I would just give away.
If you like my work, I'm super appreciative and grateful. If you like my work so much that you want me to do something for you, I'm even more so. Send me a note, email, tweet, Facebook message, whatever, get in contact with me and we'll discuss what you want me to do, I'm open to doing just about anything.
But understand that my time, effort and energy will come at a price.
Listening to: http://www.nodicecomic.com