Why he was angry at him i forgor

Misunderstandings and lack of communication. MC has suffered from an unrequited love for years and while ML pretty much acted like a boyfriend, it wasn't in a very healthy way and neither of them had the guts to label what they had even when they'd get jealous, clingy, etc. MC finally had enough of the endless back and forth and decided that if they were never going to decide what they are properly, they should simply be nothing at all.