Time and loneliness can also help you forget about your differences, even if they are really still there. That's why lots of people go back to their ex-significant others.

What >>4 said plays a part too. Lots of men and women go back because they've shared intimacy with this other person. Especially if it's been a long relationship. The other person has become a part of your life and your life can seem like it's missing something without them. This doesn't necessarily make going back to them a good thing, it just means that they became such a part of your life that you're having a hard time letting go.