If he went back to St. Mungos he would truly lose not just his magic, but his mind. As if Azkaban hadn’t done its damage. But if he stayed with Hermione, he would lose his pride. He couldn’t help but despise Hermione at that moment for wielding so much power over him. So tauntingly he said, "And what high do you get out this? Isn’t having Potter and Weasley at your beck and call enough?’