Many dogs become aggressive because they are not comfortable around people. They may develop enough confidence to be near them but do not want to have any social pressure put on them to interact. So while a muzzle will protect those from your dog it does not protect your dog from unwanted advances. A good trainer that uses positive reinforcement training techniques can teach you all about triggers, thresholds, counter conditioning and desensitization. But it’s important to understand that any behavior modification you hope to effect with this dog is going to take time, energy and patience. It’s a 24/7 commitment to management and training. If you can find someone to help you with it it can be incredibly rewarding, but only if you’re into it.