This might sound weird, but what if part of the problem is that you're too good a fit for each other? That can be a scary thing to find when you're pretty young, and different people deal in different ways. (Not that you were thinking about marriage or anything, but even a comfortable relationship that looks like it could last a long time can be scary, particularly if you're in a time where you may well have lots of changes and decisions to be making- you start second-guessing how much you should weigh the relationship against other stuff.)

I mean, one of the bazillion reasons why I am not currently even looking for a relationship is that I know what I'm like- I'll give 110% to something that means a lot to me, and right now it's better for me to just focus on what I'm doing at college and not get involved in a relationship which could be a distraction from that, or could cause me to make decisions other than what was the BEST for me. (Like you might think twice about taking up an internship for 6 months 3000 miles away, if there's something almost as good close by, plus your significant other.)

(Which is, of course, why I just adopted a new dog. Who was supposed to be another 'family' dog but who is pretty much glued to me. At least he travels in the car well.)