Plagued with fish thoughts all night so I just had another idea, could also be applied to the post-canon AU you have Pre-de-fishing, Sebastian does lean more towards the fight or flight response to danger (he has the teeth, claws and strength for fight, and the speed and mobility for flight), but the human in him has the freeze/fall instinct where he'll just drop to the ground and look as small and inconspicuous as possible, which is impossible when you're a giant fish man. He thinks it'll get better once he's mostly back in a human body, but instead it gets worse because now he doesn't have the traits he had for a fight or flight response, so all he can do is just freeze, as disgusting as humiliated as it makes him feel. He can't help it.
