Brock is Ash's repressed sexuality. He fell into the coma a virgin and needed an outlet for it. Since he can never experience it, Brock must never succeed. Brock is a projection of his sexuality, and is constantly shot down because Ash could never “know” sex. Brock isn't just Ash's latent sexuality, he's also his fatherly instincts, neither of which Ash can come to terms with. Brock leaves his siblings to join the journey because Ash can't cope with having that much responsibility, much as his foray with a real relationship ends on mysterious terms. Ash just cannot handle commitment at his mental level.