Schnauzers are, well, yes Schnauzers. It's my understanding (and I am sure the board will have no hesitation correcting me if I am wrong) they are terriers and terriers challenge other dogs with a stare. Jack may well know what is coming. He's also got the guy thing going on. I've had a male dog that would have nothing to do with other male dogs. So far two females have worked out fine for me.
OK so that's my ideas... Not very helpful but... (your signature says "attitude is everything" and Jack doesn't like this pups attitude.) OK so that makes my post even less helpful.