I've recently experienced this! The problem is that broken people, who haven't delt with psychological issues from their past, do this. I could be anything from trust, depression, rape, molestation, abandonment etc. my ex was molested as a child, and kept it suppressed for years. These people need professional help, but don't realize it. When they go through the
Ow stages of depression, they attract other people that are familiar. The Ex that he went back to, most likely is a broken person as well, and they go back to familiar/comfortable settings. They actually have low self value and are uncomfortable with people who show them the Love that they really want, but they don't love themselves enough to know how to accept it.