He put distance as soon as Aziraphale pushed the idea of coming to heaven on him again after Crowley's confession. he already knew this was over, but when Aziraphale stops him they take second shots at explaining themselves to each other.
"we can be together!"
This is when Crowley tenses up, even if I cant for the life of me find a gif of it. Aziraphale thinks he's saying the same thing Crowley was during his speech about going off together, but he isn't.
"I don't think you understand what I'm offering you"
"I think I understand better than you do"
Crowley knows they couldn't really be together in heaven, not the way he wants. They could only be colleagues, essentially go back to the way they were working on Earth before. Sanctioned this time, but still just the same.
"you idiot. we could've been. us"
Crowley starts trying to describe exactly what the difference is. no nightingales, we could've been us, anything of their coded language that might get through to Aziraphale. but everything falls just short.
So he tries just what he did in the first episode. "just breathe, breathe and count to ten. that's what humans do"
he tries the human way.
it still doesnt work.