Taking care of a human child is a fun experience for J'on, especially when that human child belongs to Bat//man. "You are most certainly more energetic than your father..." The Martian mumbles while you're busy playing with your toys and watching TV. First, he was hesitant to take care of you. Yes, J'ohn used to be a father on his own planet, but human babies are far too different than Martian babies. Unfortunately, Flash wasn't available to babysit for Bruce, and Bruce needed to leave somewhere with his butler. At last, the alien accepted, and he doesn't regret it so far. You've been a good company.