Well...I have certain perameters, you might say, for my avatars. First of all, I usually choose ones that are of darker tones - more blacks and reds, to be specific. And they ALWAYS have to be either Spike or Faith, or some kind of miscellanous thing. And they HAVE TO HAVE TO HAVE TO match my banner, kthx. This, of course, would be my OCD working for you.
I tend to stay with darker tones in both my banners and avatars because I like to give an accurate depiction of who I am - I feel like things like your signature, avatar, usertitle etc. are kind of like clothes and makeup and that sort of thing. They give you an idea of who someone is without ever really having to speak to them, or in our case, read their posts. It's something to identify someone by.
I want to be as up front as possible with people, so I go out of my way to pick avatars that kind of reflect the person I am, as much as I'm able. So, yea, my av is kind of a warped extension of who I am, at least that's how I consider it.
Also? I get bored rather easily. I change my av relatively quickly, because I just CAN, OKAY?!
...and maybe I'm crazy. Or stupid. But I'm tired, and maybe I missed the point, so I'm just going to go to bed now...