He practices all that he can, trying to make it sound like he’s really a wolf. It doesn’t, of course, but Dean assures him that he thinks it sounds just perfect. That’s all that really matters.
He thinks he’s getting pretty good at it, howling louder, proudly. Until John comes home and immediately snaps at him to shut up, stop being so silly. Sam ducks his head, ears pinning back in embarrassment.
Dean comes to Sam’s defense immediately of course, telling John that it’s a way to ensure they know when Sam’s in danger, and assuring Sam that his howls sound great.
Sam relaxes a little. Except one day, when he can’t find Dean in the parking lot after school, he doesn’t think before howling. He just means to help his brother find him, but he draws the attention of every predator around.
And every single one of them immediately advances on him for being an ‘impostor,’ and accusing him of trying to pretend he’s a predator, and they tell him he’s not good enough because he’s just a little fawn.
Dean finds them and intervenes before the others get a chance to actually do anything, warding them off. He tells Sam not to worry, that they’re just dickbags. But Sam doesn’t howl anymore, not unless he and Dean are completely alone. He just yells if he needs help on hunts, because he thinks, even when in danger, that John would be upset with him.