Here's my theory. I think that Crowley was actually an angel who fell to earth (something to do with Naomi, maybe) and then made a deal with a crossroads demon, but because it was so long ago he was the first one to ever do it. So then someone like Azazel answered, and gave him his wish and ten years, and then when he died he came to hell and took over the crossroads, hence why he is King of the Crossroads
"I told you I can handle this," you shout. "Just call it being extra careful," Crowley tries to convince. "Since when have you been extra careful?" you ask sarcastically. "Since you've become integral to the success of hell's doings," Crowley explains. A pause. "You like me," you tease. "Shut up," Crowley scoffs.