The first time Sam calls Castiel's name, he isn't even aware of that. He is just trying to do the ritual to help Bela and get rid of the nasty ghost.
What he isn't aware of either is that it isn't the first time when Cas heard his soul's voice. Castiel is used to Sam's prayers by now. It always amazes him how pure and full of faith is this boy with tainted blood - The Abomination, how the other angels see him.
Nevertheless, Sam asking for HIS help is like receiving electric current to his grace and Castiel feels inexplicable need to hear his name falling from this human's lips again.