Love is a powerful emotion, but sometimes it's not powerful enough. Tatsumi loved Tsuzuki deeply, but that didn't keep them together.
Tatsumi knows he let Tsuzuki down when he left, knows that he made a piece of the grief that threatens to rip Tsuzuki apart.
It's that knowledge, combined with the shadow of the love that he once had, that keeps Tatsumi protecting Tsuzuki, a job that is neither easy nor comfortable.
It's that knowledge and that love that keeps Tatsumi from ever seeking anyone else out, ever finding peace again. Keeps him from loving Watari.
Guilt is powerful, too.