[Billy looks startled by this revelation; he's never understood his inability to heal his own injuries, attributing it rather pessimistically to his stubborn old sense of self-loathing, but that... made an astonishing amount of sense.]

I never thought of it that way... but yeah, that's probably it. Though I know I can't heal people that I... don't really like. Or have a reason to be angry with them, I guess.