Also , Sam's shocked he's even saying any of this . " You want to go back to how it used to be . " Sam answers for him , and kind of gets it because he remembers when they would just hunt for the sake of hunting - not because they were trying to find their father or because they needed to kill a demon . When they were teenagers , back then the demon was sort of this thing at the forefront of their father's mind and more or less at the back of theirs . 
Question: Why did they hunt for the sake of hunting ?
Because they enjoyed hunting .