What happened to me when I was younger, and I've seen it with many people I know, is that when you meet the first time, you are both single and doing interesting things with your time.
You fall in love and start spending your time with each other, and stop doing these interesting things.
You eventually start to grate on each other, find the situation a boring, dead end, and break up.
Lo and behold, 6 weeks later you bump into your ex in some bar and you and they are the interesting, vibrant people you were when you first met. So you date again. And the same thing happens, only quicker. You just have bad chemistry together and stop being the best person you are capable of being. Them too.
Part of all that was also immaturity, the tendancy to fall totally in love when I was 20 or so, and the willingness to give up so much my life to hang out with my GF. I wouldn't live that way now, and the women I meet wouldn't want that either.