Because, despite the fact that they both basically just want to love each other, there's always going to be that schism between how Dean thinks he can do that and how Sam thinks he can.

That's very true. Although, being a sap at heart, I like to think that someday they'll be able to come to some kind of compromise about it. Sam's at an age when he's pushing so hard to establish Who He Is, but it will mellow. And as parents everywhere must do, I like to believe that Dean will one day be able to see that he can let Sam go and still have him. *is all optimistic*