Despair. Fourth stage of grief.
Perhaps Aziraphale didn't want anything more to do with him anyway. Now that Heaven was accepting him back and he was going to achieve the status of supreme archangel, Crowley no longer interested him.
He was just a demon, what did he expect? Angels and demons never joined forces for eternity. Beelzebub and Gabriel hadn't been seeing each other long enough for this to be irrefutable proof.
And let's face it! This is all the fault of heaven, metatron, hell and even Lucifer too, for that matter. And himself... Without all his questions back then...