Crowley and Aziraphale have known each other for a little while, just a few millennia. But, something is coming, something that has Aziraphale very, very worried. Crowley can't just up and leave no matter if the angel begs and pleads, probably. This is the demon and the angel's first big falling out that will change...
Crowley can't stand to be around Aziraphale anymore. It hurts too much to have the angel he loves deny his friendship. He can't stay just friends, so he has to find a way to end their friendship, but in doing so, he may have broken irreparable bonds.
After the near end of the world, Aziraphale and Crowley decide that they are happier when together and make the big decision to move in together, which ultimately leads to a romance that has been building up for centuries.