Hey Dean, what was it like? They, uh… They sliced and carved and tore me in ways that you… Until there was nothing left. And then, suddenly… I would be whole again… like magic… just so they could start in all over. And Alastair… at the end of every day… every one… he would come over. And he would make me an offer. To take me off the rack… if I put souls on… if I started the torturing. And every day, I told him to stick it where the sun shines. For 30 years, I told him. But then I couldn’t do it anymore, Sammy. I couldn’t. And I got off that rack. God help me, I got right off it, and I started ripping them apart. I lost count of how many souls…The – the things that I did to them. How I feel… This… inside me… I wish I couldn’t feel anything, Sammy. I wish I couldn’t feel a damn thing.

4x01 “Lazarus Rising” 

I don’t think this scene gets enough notice. Robert singer, the man who stabbed his demon possessed wife and had lost damn near everyone he ever cared about had finally got something good, he got dean back, someone who he considers a son. bobby had clearly long given up any hope of seeing dean again, he had lost all contact with Sam and was left alone, drinking his sorrows. When he first sees dean he attacks him, not even considering that it could be him. Because Bobby singer had lost far too much to let himself believe in a good thing happening to him.