Guilt. Second stage of grief.
Why hadn't he realized his feelings for the angel sooner? Had he really spent the last few thousand of years believing they were just two entities getting along "pretty well"? Whereas he never felt that much alive than when he was in their presence.
If he'd realized sooner that what drew him to the angel was not just curiosity, but a deeper feeling. Something about love and not the kind God wanted them to have for their new creation, but their own.
If only he had opened up then, it would have been easier! Idiot.