You're a demon. So, of course, Dean doesn't trust you in the slightest bit. Just like he doesn't trust Ruby. Yet, regardless of Dean's undeniable hate for you, you want to help the man. Sure, you're sadistic and a bit psychotic. You're a demon. What did you expect?

Dean knows about Sam working with Ruby and his use of demon blood. That's the only reason Dean reaches out to you for help. You're a demon, so he assumes you might know what Ruby is up to. Even though he doesn't trust you in the slightest, he trusts Ruby even less. So, in a time for a desperate need for answers, he calls you. And when you show up, you meet him outside the motel room in the middle of the night...