Everybody sing with me! There is a house in New Orleans they call the Rising Sun!
All he ever wanted was his brother. He didn't need ghosties and ghoulies and things that went bump in the night. He hated those things.

Sam was, technically, one of those things.

Sam couldn't think that Dean hated him, though. That would be bad. That would give him an excuse to run away and never return.

So Dean put up with Molly, even if every fiber of his being told him to shoot her. He was going to prove that he could accept ghosties and ghoulies and things that went bump in the night, as long as it meant that he got to keep Sam. It was because of this that he knew he was selfish.