"It's not. The angels that become human forget the things that can't be contained in a human mind, but they won't forget that they once knew."

Things like what Heaven looks like to the angels, the face of their Father, the faces of their brothers, the sound of the celestial choirs. Important things, and there's little more painful than knowing that you once had a home, but not being able to remember any of it. They don't get the luxury of forgetting everything, unless they do as Anael did and tear their Grace out before they fully Fall.

"I always thought it was a particularly cruel punishment. The angels who are cast out for disobedience and become human are just as mortal as any other human, and when they die... let's just say that Heaven holds grudges."