Demons, by definition, were evil things. Cruel, merciless. And a demon was created when an angel Fell, corrupted. As such, they lose the ability(and, quite possibly, will) to love or care for another.

But Crowley did not Fall so much as Saunter Vaguely Downwards.

And he was the only one that had ever Sauntered Vaguely Downwards, while countless angels have Fallen.

The rules of the game are much different when you Saunter Vaguely Downwards. You don't lose the ability, but you fail to see the point in loving or caring or any of those frilly emotions. You also have a much higher level of hesitation to admit that you'd ever feel these things-after all, you are a demon, and demons, by definition, aren't supposed to feel such things. They were supposed to be ruthless. Conniving.

But no matter how much he tried to deny it, or force it away, or something like that, Crowley did have these frilly emotions. Less so than if he'd been an angel, or human, but he did all the same. And that was quite an accomplishment for demons, and more so of an accomplishment for Crowley.

Being around an angel for about 6000 years helped, too. Good influence and all that.

It also helped that said angel was the one he loved. In a way. In another way, this was terrible, because that angel could so easily see it. Heck, that angel could probably feel it. And it almost gave Crowley shivers. Almost.

But not quite, because it was also a relief that he'd never have to say it out loud to make sure he knew. Crowley knew that it would probably physically hurt to get the words out, and even when he did, it would probably be stuttered and his voice would probably crack, and Crowley wasn't about to let that happen.

Which is why, when the angel heard him speak it out loud, that he'd nearly fallen over. He said it flawlessly, not a single break in the words or any sort of indication that it'd been forced. But the angel knew not to make a big deal of it, and simply smiled at the demon and returned the words, in the back room of a bookshop, a bottle of wine between them, the smell of ancient books and yellowing pages around them.

And the demon smiled, and took a sip of his drink, and they continued on in their merry talking, becoming more and more drunk with each glass of wine until they were laughing and they had no idea what they were laughing at.