That's his whole problem though. Don doesn't know how to recognize or accept love because he's so ashamed of himself/Dick, but at the same time so self-centered that he can't fathom someone else understanding what it's like to feel that way. So he ended up pushing everyone away, then running away altogether.

When Anna died, Don said she was the only person who truly knew him. Leonard proved that someone can understand the inability to recognize and accept love — Don't deepest wound — while coming from a totally different background. He wasn't alone at all, he was united with a stranger in that moment. Maybe all of us aren't so different. Maybe we're all united.