Most Helpful Guy
In reality the only one who really knows that answer is him. We can only speculate an opinion based on what you're telling us, but it is subjective at best. He would be able to divulge the facts.
I want to say it's because of the baby, but the fact that they keep breaking up and getting back together inclines me to believe that while it may be in part because they share a child, but I think that he really still has some sort of feelings for her as well and is trying desperately to hold onto her. His returning always comes with the happiness that they're back together and the hope that "this time" they can make things work.
Of course, either she, or he, or BOTH of them contribute to the breakdown of the short lived bliss they share as no one seems willing to communicate and get to the root of the problem(s) that always seems to drive them apart.
The result, as repeated several times before, never changes and inevitably they break up again, never really finalizing any decision(s) and never really completely getting over one another. Sure they go about their lives and see other people but something (the child in particular) keeps making them gravitate back toward one another.
This is a very volatile relationship to say the least and it is very unhealthy for all involved ESPECIALLY THE CHILD. Unfortunately, it's their selfishness and their magnetism which keeps drawing them together will continue to fuel an endless cycle of an unstable relationship and unless one (or both) change in effort to either stay together or to decide to part and never get back together again will only keep them on this vicious cycle until somebody takes a stand and stops it.