I'd agree with the thought that girls cut their hair to mark events. A friend of mine cut her hair for her 21st birthday. I, on the other hand, love keeping my hair short all the time. At the moment, it's about 2" long on top and 3/4" in back, so I don't think I can go much shorter.
Though I went back to spiking my hair after a rather nasty breakup with a b/f, primarily because he hated my short hair, and I felt like harassing him visually when we were in class. Common consensus I've heard is that guys don't like girls with short hair - even my female friends agree. Any comments on that?