Why is my chihuahua so mad at me?

Dogs dont feel the resentment that you think you are seeing. your dog is not mad at you because you got another dog. Sounds like manny is letting everyone else know to back off while he is getting attention. If manny was really mad at you, he wouldnt turn away. That acton is agressive to others. I dont recomend rewarding this behavior with praise. Send him away if does this. He will eventually learn that his actions are not ok.