The major problem they have more than anything is that Erik has been so badly burned before, by basically everyone, that it's like he's always waiting for the slap. He can't quite trust Charles not to change his mind, or to be doing things for ulterior motives, because nobody else ever has.

And Charles is stuck there, no matter how well-intentioned, and so he's trying to make sure he doesn't fall for Erik just because Erik is keeping him there. It makes it difficult for them to fully connect without each of them worrying about the other.

Maybe you should give them both a slap anyway, see if it knocks something loose :D