It wasn’t easy for Dean Winchester to say “I love you,” because, since he was four years old, he equated love with loss. His mother burned alive. His father left him in rundown hotel rooms, and he was forced to change the role he should have had - a big brother - into that of a caretaker. He lost his childhood, but he did a damn good job raising Sam - and he even loses Sam for a while too, when he goes off to Stanford.
Then Dean loses the first person he loves romantically, Cassie, when she finds out what he really does. He leaves Lisa and Ben - despite admitting that he wanted a family - because he feels all he will do is put them in danger and that he’s not good enough for this. He believes he’s a killer.
He goes so far as to ask Castiel to erase Lisa’s memories of him and he uses a car accident as a metaphor for letting down his guard - he lost control and accidentally hit her, and he’s sorry for it.
And then there’s Castiel, who is willing to make it up as they go. Cas, who dragged Dean out of Hell and disobeyed Heaven to help the Winchesters. When Cas does something Dean sees as a betrayal, he takes it harder than he does when anyone else hurts him - but he forgives Cas. When he loses Cas, he’s devastated - he makes up an alternate scenario in his head to forget that Cas didn’t want to be dragged out of Purgatory. He carries Cas’ trenchcoat from car to car.
Dean knows what he feels for Cas. But can he say it? No, because when Dean Winchester loves someone, he loses them. So there’s little touches - running his hand down Cas’ shoulder, telling him “don’t get dead.” Saying “I need you.” He makes Cas mixtapes and watches old movies with him. He shares a late night drink with him in the Bunker and then slowly makes his way into his bedroom, alone, wondering why the hell he can’t have one goddamn thing he wants.
Then one day, Cas tells Dean “I love you.” But it’s not in the middle of the night over a glass of whiskey after some hunt like Dean wanted. The world is ending and Death is at the door knocking, and Cas tells Dean that he loves him but that this is a good-bye.
Yeah, Dean couldn’t reciprocate. Because in that moment, all he got was confirmation that love means loss. What do you say when your biggest fear comes true?