My dog is aggressive towards other dogs?

My chihuahua/Jack Russell mix seems to like people and car rides better than his own species. He does this to all dogs, and I tend to avoid other dog walkers when out at the park or on a walk with him. The two dogs will sniff each other, then my dog will start growling. Usually the other dog will think its a a game and get excited, but then my dog will snap and growl more. This is usually when I pick him up before any trouble starts. He seems to do this less when he's off leash, but he will do it to certain dogs even when he is. I'm just worried that things will go too far.

I have a jack russell and she has the same sort of personality. My dog is fiercely protective of me and therein lies the problem.In my experience, this is why your dog is behaving this way when on lead. The fact that you pick him up makes the problem worse because you are elevating your dog higher up than where the other dog is. Being picked up may also be reinforcing that he is displaying the right behaviour in this situation when in fact it is the wrong behaviour.I think leaving him at ground level and correcting him would work better but don't force him to be buddies with the other dog if he really doesn't want to.Notice how he behaves a lot better when off lead? There is not much you can do when he doesn't like certain dogs. My dog is the same. I just need to keep an eye on her and watch for when she looks like she is getting upset. Some dogs she will happily go up to and start licking them in the face. It's bizarre to say the least.I do have to say that because I don't trust my dog 100% I do not take her to the dog park. I do walk past the fence there and let her say hello when the other dogs run up but she chooses who to rub noses with.I have not fully correct her behaviour because she is not always in contact with other dogs. Mostly it's when I walk her that we stop and say hello. I have noticed that she will bark when other dogs approach (again, I think it's the protective behaviour) but if I stop and chat for a while, she is quite happy to be around the other dog.If you really want to nip this in the bud then I would suggest getting in contact with a dog trainer/behaviourist. They will be able to tell you why he is acting this way and then what steps to take to properly correct it.One more thing I would like to add. Get him to the vet for a good check up. My dog really was grumpy for a while and growling at the other dogs in our house even. It was a bit out of place so I took her to the vet and discovered that she had a bit of a back injury. Probably because she got bowled over by one of our larger dogs. She underwent some acupuncture treatment and is okay now. So her growling problem was because she was in pain and didn't want the other dogs going near her. A dog in pain will certainly act this way and be grumpy.Just a thought but maybe worth a quick check to make sure nothing is going on with your dog that you don't know about.