Lil Angel

I've made a short clip of the important parts of the scene:

Cas deadass told Dean exactly on how the Empty would take him.

Cas explained to Dean that true happiness can happen by just him saying something to Dean; "Happiness isn't in the having.... It's in just saying it."

Dean asked "Why does it sound like a goodbye," and Cas said "Because it is," so when Cas said his i love you, Dean said "Don't do this, Cas." because he knew that Cas was gonna sacrifice himself to save him.

His face after Cas said that it was a goodbye:

His head turn at the last minute, you can tell that it was a "Wait, no..." or "It can't be..." look. The reaction of someone who had connected the dots all together. The reaction of someone who finally understood something.

Dean's reaction after Cas said I love you:

That is a reaction of someone who is about to ugly cry but is trying hard not to. Dean knows Cas is gonna sacrifice himself.

Basically, Dean's "Don't do this, Cas" actually means "Don't leave me. Don't sacrifice yourself for me. Don't tell me that you love me & then die for me. Don't do this to me, Cas."

Supernatural 15x19 — Inherit the Earth