Anger. Third stage of grief. And coincidentally, one of the 7 deadly sins.
Crowley could only think of the last words they'd exchanged with each other before they parted for good. How could he ever have dared even suggest that he'd be better off if he became an angel again? Up there? With that bunch of... RAH!
And besides, what? With all the sweet gestures Aziraphale paid him, their gentle smiles, their gazes... Could they have initiated certain things on their own? They could have realized! What an idiot! They could have been them! A couple. Like Beelzebub and Gabriel.