How often do you develop a crush on someone?

I've noticed that a lot of my friends (aged 20 - 27) have crushes pretty often, say one every 6 months. If they are single, of course. I'm 24 and I've only had a crush on someone two times in my entire life, and it gets pretty boring when you don't have anyone to look forward to or fantasize about. I meet enough new people, I have about two dates a year, I just never feel that attraction toward anyone.