Dean Andrew Winchester was tired and depressed. Unfortunately, he was taking it out on his younger brother.
He really didn't mean to, but all he seemed to do lately was hurt Sammy. He knew about Sammy and the bathtub thing, but he didn't care when he snapped and told his brother it was his fault about Mary's death.
He didn't know about the last fight Sammy and John had. He probably would have been angrier at his brother if he found out what had happened.
Dean just wished that his brother would stop bothering him about his grief. Dean was dealing with enough on his plate, and his brother was just annoying him.
John's death had devastated Dean, but it had also made him angry. Why did John have to die for him? Why couldn't he have let Dean die? Sam didn't seem to understand that. Sammy was being selfish and only wanted to deal with his grief in his own way.
Dean had always hated it when Sam retreated to a bathtub. He usually ended up yelling at his brother to get out, but it never worked. "He's just dealing with things in his own way," John told an eleven year old Dean when a seven year old Sam retreated to the bathtub because of his pet hamster dying.
Dean loved his brother, but he just wished that things were different. When he feeling really low, he even wished Sam had died instead of John or Mary. He immediately felt guilty afterwards, but he was past the point of caring.
Dean just wished that he was normal, and he resented Sam for the fact that he got normal for four years. He also resented Sam for the fact that he never said anything when he had started crying over John dying for him.
Unfortunately, Dean would come to regret the fact that he was no longer paying attention to his younger brother. Things were about to get bad.
Very, very bad.
TBC