Subaru and Kamui are lovers. Or perhaps, not lovers, but simply together. Subaru is a consolation prize for Kamui, who is forever without Fuuma; Kamui is Subaru's one attempt at a healthy relationship.
Subaru gives himself up to Kamui, a willing sacrifice to love that doesn't exist between them. They pretend so well, these days.
And then Subaru speaks, and Kamui can never believe the words. They are simply there to thank him for the sacrifices he has made, and the worse ones he will make. The words are meant as healing.
Well meant, but meaning nothing.
"I love you."