My wife and I became foster failures when we adopted The Dude (2yo Russel Terrier/beagle mix) 6 months ago. He has been very gentle and sweet with most people and very friendly and playful with most dogs. Lately however, he has shown aggressive behavior toward certain dogs and my wife. We have had a trainer come by to work on greeting another dog with our neighbor and her dog and have been giving lots of tasty treats that he only gets when other dogs are around which helps with most dogs, but there are certain dogs, that no matter what kind of treat we try to give him, he goes nuts when he sees. He pulls, barks, growls and tries to bite when he gets anywhere near them. Over the summer, we spent a lot of time at the dog park, but stopped going because he would stop playing nice when certain dogs showed up. Also, he loves my wife and snuggles up on her lap and enjoys having his belly rubbed by her, but when I am around he will show her his teeth and growl at her. He has even nipped her lip when she didn't back off. She has had success when giving him treats before interaction, but doesn't want to have to always have treats in order to pet him. Any advice?