Commitment issues, baby! :-)
Sorry for being flip, but I've seen that over and over amongst my friends (yeah, just my friends,
I never had this problem... ahem...)
It's a way to go through the fun and excitement of dating with, counterintuitively, less risk. If the available one is available, you might actually end up in a
relationship!
I think you said you're just 21, so that's less of an issue than if you're 41 (but still only an issue if being in a relationship is a goal -- if you WANT to be single, more power to ya.)
What has happened after things are "fine"? How long has it lasted then?