I love it, because Dean is so painfully dense that he comes to the conclusion that Sam and Castiel are having a Secret Romantic Relationship. Initially, his first reaction is to be upset and jealous because clearly he met Castiel first. There are rules about this. But then he sets his own feelings aside and tries to be happy for them, dropping hints to Sam about the things Castiel likes while wallowing in -what Dean thinks- is an unrequited love.
Sam and Castiel, to say the least, are confused by his actions.
Apparently, I have a thing for longing fics. Fics where two characters obviously have feelings for each other and are just so right when they’re together, you want to grab and shake them when they’re being clueless to how they feel about one another. I think that’s why I enjoy reading The Soul Piece (my rec is here) by cloudyjenn. The way Castiel and Dean are literally made for each other but are forced to be separated. There’s just something about it that hurts so good. Because clearly I have issues.