Dean does his best. Calls his brother Sam. Treats him like an adult. Talks Dad into doing a few hunts on his own so he can stay home and Sam doesn't have to come with.
Dad and Sam are pulling him in two. Sam's need to stay home, stay in school versus Dad's need for help, backup on his hunts. It kills Dean to leave Sam, though Sam's disdain for his big brother's protection is clear. And it kills Dean to stay with Sam, knowing that it leaves Dad unguarded, more vulnerable when he's alone.
His best isn't good enough.