Katsuki recognizes his love for Izuku isn't healthy, it's something toxic, closer to an addiction. It scares him. He's terrified of how much his mood and goals depend on Izuku.
He almost doesn't feel like his own person, it's like he revolves around Izuku. It scares him enough
Mina crying over getting dumped and Katsuki says "why would you ever cry over a man?" and Mina stops crying to glare at him like "I know YOU aren't talking"