I have a 6 month old male kelpie x border collie. He's an obedient dog at home and well trained, however when we have visitors around he gets overly excited and difficult to control because he just wants to play. He is the same with other dogs but 99% of the time this is when we're walking and I can pull him back on the lead to control him. However a friend brought her dog around to meet mine and my dog got very aggressive and growled at her other dog. He's never shown this kind of behavior before and I'm not sure why. I need advice on how to control my pup and get him to listen to me when he is around other people and other dogs, it's like I don't even exist! Please help!!!!