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S2 and S3 are halves of the same dialogue.

Everyone's saying how Crowley "choosing humanity" over Aziraphale feels OOC—usually it's the other way around (that's how Crowley got stuck in his infinite yearn loop). In S2, Aziraphale faces this ultimatum where he can either choose C or Heaven, and he chooses Heaven. He also tells C "I forgive you", which can feel like it came out of nowhere. Why?

Notice how Aziraphale PAYS for his choice in S3: Mrs. Sandwich has a go at him when he's back on Whickber St; God calls him a few variations of "selfish dick" at the end. "I forgive you" was meant to feel like it came out of nowhere because it was a culmination of A's hurtful, subconscious microactions against C.

Aziraphale realises this at the end of S3. In apology, he sacrifices Aziracrow Eternity to ask what Crowley wants. He expects Crowley will want the same thing but—in a seemingly OOC move—Crowley picks humanity.

Which is Crowley's answer. By choosing Aziraphale, Crowley would have returned his "I forgive you", and Crowley does not forgive him.