When Pierre is happy Andrei is troubled, when Andrei is darkened, Pierre is giddy. No wonder they are friends: opposites attract.
It is just like Pierre to be innocent enough to not even know why he’s unhappy. The woman (well, girl) he loves will marry another, but he doesn’t even realize he loves Natasha. It never occurred to him he loves her, but he feels the pain of not having her just the same.