He is still friends with exes who he had terrible breakups with—breakups that involved screaming, constant fighting, atrocious acts, and perhaps even cheating. There is only one reason a person remains friends (if you can call it that) with an ex that he has such a turbulent past with; there is something he still wants out of that person. Whether it’s to make her jealous, or to make her love him again, in some way he still deeply care what that person thinks about him. He’s still trying to win the battle that never really ended, just because the relationship did. And that’s easy territory for cheating.