"Oh … oh, my dear …" Aziraphale gasps as he relaxes underneath his husband, legs spread and back arched while Crowley's mouth travels down his skin, seeking out every vulnerable spot with his tongue and stroking over lightly. Crowley loves hearing Aziraphale sigh my dear almost as much as he enjoys hearing his name pass over his angel's lips. He's been my dear to him for so long.
But not quite as long as Aziraphale has been Crowley's angel.
Crowley stops kissing at the center of Aziraphale's chest, turning his head and pressing an ear above the angel's heart, feeling the thrum of it beat against his cheek. Crowley always thought his heartbeat should match Aziraphale's. That's how the poets and the romance novelists make it out, anyway.
He was distressed when he discovered it didn't.
Long nights Crowley spent after the two of them had gone to bed with a hand or an ear pressed against Aziraphale's chest, waiting for their hearts to match up. After they made love, he'd press their chests together in the afterglow, hoping their hearts would talk to one another, come to some consensus over meter.
But they didn't.
And because of this, a sort of self-doubt bloomed, along with the ridiculous idea that they weren't meant to be together simply because an electrical impulse in Crowley's mortal form (which, admittedly, he could change at any time) didn't line up with that of Aziraphale's.
It was asinine. Crowley knew. He was looking for needles in haystacks already packed to the Heavens with needles.
Still, he spent too much time considering it, too much energy thinking on it. He became an irritable, over-anxious, paranoid mess - not too different from his usual irritable, over-anxious, paranoid mess, so Aziraphale never thought to ask what was wrong.
It was on a cold winter day, their first Christmas together, that Crowley finally figured it out.
Crowley's heartbeat doesn't keep pace with Aziraphale's, doesn't mirror it.
It echoes it, follows it the same way Crowley has followed Aziraphale for hundreds of years, chasing after and then slowing down, much calmer when it finally catches up.
The same way Crowley had always intended on following Aziraphale, no matter where he goes, no matter what trouble he gets into, for the rest of their existence.
Which is why Crowley couldn't leave Aziraphale on earth and go off to the stars alone.
Without Aziraphale's heartbeat, Crowley's echo could not exist.