Oh, Dean. I think Dean often doesn't see Sam's issues, sometimes because there are resentments he's hanging onto (*cough* Stanford *cough*), but usually just because he loves Sam and most of the time he thinks Sam is a good guy and it doesn't really get through to him that Sam doesn't think Sam is a good guy, or that if he has doubts they won't all go away if Dean reassures him. But if Dean does get a grasp on a particular issue of Sam's, he'll totally go into fix-it mode on it and be the cheerleader who's sure Sam can work through it. Which is really ironic, given that Dean is sure that Dean's own issues are unfixable. Boys, so awesome, yet so stupid.