Also, he had no clue where to start looking because he had no idea where Dean went or if he was even alive. He thought Dick Roman was obliterated and so might have been Dean along with him.

Sam tries so hard to be independent but for all of it, he needs his brother to truly cope. He can move on, he can find other things and occupy his time but Dean is rope that keeps him tethered.

Everyone says about Jess and his time at standford but really if he was truly happy at school, even after Jess he would have gone back and resumed a life away from hunting which he professes to hate to much. He could have left Dean and had a normal life again but it was Dean that kept him from doing that.

Sorry for the long meta reply.