"Course he's jealous, have you met me? Everyone's jealous," John mocked because he couldn't help himself. His go to response was arrogance and while that often shot him in the foot, he doubted he'd be able to come and out play anyway. The Doctor didn't trust him because, well, he was a prick. A total prick. And John wanted to shoot his kneecaps off.

That was probably why he didn't trust him.

"I'm always useful, you know? I can help. I can even be good. And like I said, unarmed. See, I'm harmless. Harmless as a boy scout."