When Cas comes into heat, he figured out that Dean is his mate. Cas is afraid to go near him, because when an angel is in heat, their wings become visible and have effects on humans unlike any other. Dean calls for Cas, and finally, Castiel cannot resist.
After Sam and Dean have an argument and go their separate ways, Cas spends a night at a bar with Dean. They find themselves drinking and getting closer and closer to the point that when they go back to the hotel, Dean wants Cas to stay. And so he does. Dean confesses his love for Cas and Cas admits he has the same fee...
Dean always knew he had an attraction for guys. He was just too afraid to embrace it. That was until he met Charlie Bradbury and she helped him to tell the ones he loves.