But so is Rei.

He was willing to step aside if it meant mending their friendship, and instead of acting hurt or jealous, which would've been completely acceptable, he's proud of them, and happy for them. Because he understands Rin, and all of them better than anyone else, he was able to watch them win, without him, and still smile sincerely all the while. If you ask me, Rei really is the most beautiful of them all.