Why does my dog growl at other dogs when on leash, but when pulled away whines...

[Dawnwake]

...because he wants to go to it? I live in an apartment with my dog, and everyday we take him to my father in laws to play in the backyard while we're at work. He visits with other family members dogs and now with my father-in-laws new puppy. Everyday we walk to and from the car on the leash, but sometimes there is another person with their dog and my dog will start to growl at it. Automatically I pull him away from the other dog but then he starts to cry. I don't understand him at all, why does he growl aggressively and then WHINE because he can't go over to it? When I allow him to visit the other dog he doesn't try to attack it, he simply smells the dog and is usually calm except on one occasion at my mother-in-laws when he got in a fight with her other male dog. I just don't understand, is it just fear? When strange people will come over he will growl, but be wanting attention at the same time. It's sort of a mixed signal.