Dean misses the way things used to be. He misses the world, misses his car(God, the Impala; what happened?), misses his brother(he knows what happened there) and he knows he's not the only one. Cas had made no secret of it, but he was dealing; they were both dealing. And then he came along. Dean from 2009.

Past!Dean is a knife through the heart; he sees everything he used to be and that's not even the worst part. The worst part is Cas sees it too and Cas likes it; he likes the old Dean. Even tells him so and Dean has never hated himself more than in that moment. Because if he's hating his past self, he's hating himself and that's a whole mind fuck he's not dealing with.

Dean needs his past self to see this, to change this. Because if the past can be changed, then this future won't exist and Cas won't have to look at him through that drug filled haze; maybe Cas could look at him like Dean imagines he used to. Like a savior, like a friend, like. . .

Dean wants the happy ending, sure; if the world can be spared, that's great, but he'll settle for any kind of ending as long as it's him and Cas(and Sam looking uncomfortable beside them). And maybe he'll have it one day.