I need to compare these face shots
This is what I see:
The top gif is full of duty. Dean’s face is set strong, he’s not going to fall apart because he knows Sam couldn’t handle it. He’s being the parent, putting on a show. His arm stretches across Sam’s shoulders protectively and he kind of vice grips him.
But, in the second gif, Dean’s face is soft and full of regret. He’s easier with Cas. More open and vulnerable in this shot. There are things unsaid there, a million thoughts and nothing he can do about it, because he’s all out of time. He doesn’t vice grip Cas. His hand is dead center on his back, pushing their chests together. It’s a warm, intimate hand placement, and he just kind of holds him.
Really, his face has love in both gifs, but I think it’s two different examples of love. Familial versus romantic.
This idea is followed through later when Dean asks Cas to look out for Sam when he’s gone. Dean views himself as Sam’s parent, and asking Cas to fill in for him in his absence is like asking a significant other to step up. It’s very, very much a “look out for our son” kind of moment (without Sam actually being their son).
Of course, this is just my musings on it. If anyone has anything they’d like to add, I’m all ears!