Why do guys leave their girlfriends who are nice and find a different girl and then figure out that they want their old girlfriends back?

Well, from a guy's point of view, you need to understand that a lot of the time a guy can either get bored with a realationship, or he might think he likes someone else more than his current girlfriend. If what you two had/have was amazing, he'll come back. Give him sometime, it will happen. I don't, however, suggest that you go out with an other guy. If this happens then once he realizes that he still really likes you there are going to be problems. These problems either resolve in fighting the other guy and then you might not take either of them back, or he'll give up completely and be completely and totally heart broken. The best thing to do is just wait it out and see what happens. You have to give the guy some time to think about having you or someone else. If he gets a different girl then you ask him to a double date with a person to make him jelous. Then if he askes you if you want to get back together than you say yes and then you hook up his ex and your ex and then everthing works out fine :) So then everybody is happy =D