When my wife and i just graduated college and dating, she said that she wanted to see other people, I was devestated. I did all the wrong things as normal people do, the flowers came, the phone calls, going up to visit her at her house unannounced the whole 9 yards. It took about 3 months to get over it enough to ask out this other girl. When we went out on the town that night, my wife's brother saw my date, who looked fine by the way, and of course I was estatic. What do you know, guess who calls the next day.
We never want to lose things, and if her back up plan (you) is slipping through her fingers, then she gets very nervous. This whole principle is exactly like what happened. If she perceives that you are going to be happy without her, her instincts are going to kick in, she thinks that this cannot be true, and she will come back and will remind you that you need her to be happy.
