Why do girls compliment me when I get a haircut?!

well you obviously don't look butt ugly then... its just a personal thing...embrace the shortness...

Without photos, it's hard to tell if they're being genuine, or simply polite. However, I personally prefer men with shortish hair, regardless of how good they may look with it grown out.

apparently you don't look so bad if girls like it.

Perhaps some girls are attracted to the more masculine look assigned with shorter hair? To them, longer hair seems subconsciously more feminine to them, making it seem less attractive?
Or they're possibly just wanting to be polite and make conversation, and seeing as haircuts can usually end up being noticeable, it's the first thing they think of as a compliment.
I honestly can't say for sure. Personally, I prefer it when my men grow their hair fairly long. =)