This phrase pops into my mind from time to time when I think about these two, and I was going to draw something with it before Nexus' death but never got around to it, I guess.
I just feel like it fits them perfectly. Of course Sun loves him, he was his moon at first after all, so he loved him the only way he knew how to. But Nexus probably felt like it wasn't his love to take, and he's just borrowing it from Old Moon. To him, he only got this love because of his proximity to Old Moon, not because he's kind, caring, and considerate but because he's more kind, caring, and considerate than Old Moon.
He never felt loved for who he is but for who Old Moon wasn't... And it's tragic because Sun didn't do that on purpose, and Nexus didn't know how to communicate his problems, only how to bottle them up until it all came crashing down.
They're gonna be the death of me, I swear...