Uke who is an extreme masochist get off on being humiliated and hurt. Seme started sex only relationship to satisfy uke... Somewhere in between seme falls in love with uke, uke doesn't fancy the concept of love, so seme due to one sided feeling not being returned, turns extreme in his sadomasochistic relationship.
Spoiler:
When uke guess that seme is not happy being in relationships he ask to break up (after breakup sex). Uke realise he loves seme and confess. Seme accepts. Happy ending.
Yoru wa Tomodachi