Kamui loves Subaru, and has for a while now. It's painfully obvious to everyone else. Everyone, perhaps, except Subaru.
Subaru loves Seishirou, which is why he's never noticed Kamui. Subaru belongs to Seishirou heart, body, and soul, so completely that he's never, since he was six years old, considered anyone else.
Kamui hates Seishirou for everything he's done to Subaru, but especially for keeping him. Kamui doesn't realize Subaru won't let go of Seishirou either.
Seishirou feels disdain for Kamui. Kamui follows Subaru around like a lost puppy. And we all know what happens to puppies when Seishirou's around.
Backlash.