Family
Crowley and Aziraphale were not raised with traditional family dynamics. This was partially because they grew up surrounded by angels and demons, as well as god herself, and partially because the people they were raised by were insufferable.
Living in hell was not fun for Crowley. He had this constant desire for validation – for someone to tell him that he was doing good at being bad – which wasn't something he got from the other demons. Apathy was often the best reaction he got from the other demons.
He had a feeling that the fall had something to do with it. Perhaps God kicking him out of hell had impacted his psyche.
Aziraphale, on the other hand, got too much attention from his peers. It was just all the wrong kinds.
The worst offender was Gabriel. He was always making snide comments about Aziraphale's corporal form or calling his ideas stupid. He would always drag Aziraphale right to the edge of snapping – right up until the moment that Aziraphale would consider doing something crazy like hurting Gabriel or worse – then he would pull back a little.
He would be just kind enough, for just long enough, that Aziraphale would think the bullying would stop. Then it would start again.
One memorable day, in the late eighties, Aziraphale had happened across a book about abusive relationships. He was startled by how much his relationship with Gabriel had in common with them.
Still, family is more than the people you are raised with.
To Crowley and Aziraphale, their families were the people they chose. Anathema, Newt, Adam and the Them, Warlock and even Mr Shadwell and Madame Tracy. They were their family.
A perfectly imperfect chosen family.