How "playful" is mack? You can't always assume that the other dog wants to play. If Mack is a up-in-your-face kind of dog, then the other dogs may be trying to teach him a lesson-a lesson he may need to learn! Lots of times w/ puppies, other dogs let them get away w/ "forcful playing", but once they reach a certain age, they need to learn that they can't always demand attention. Have you ever had a person always up in your face, being so friendly that you want to puke? Sometimes the other dogs don't see it as a welcome, they see it as a threat. Or you may see it as a welcome, but YOUR dog may be treatening the other, and the other dog knows it. Body language in dogs is the number one way to predict a fight. My advice to you: Don't let your dog become overly excited when he meets a new dog. Encourage a friendly greeting, but if he starts to get to happy, walk a way. Let him re-aproach the dog when he gets his manners back. A good way to teach this is to have something WAAAAY more interesting than the other dog (ei: cheese, a toy, other yummy things), then Mack learns that sometimes a new dog on the street isn't the most fun thing you could be doing, you could be EATING! Be very careful at the dog park (if you go) not to let Mack charge up to dogs. this is a HUGE threat and violation of personal space (for the other dog). Keep something yummy on you at all times so he learns that it isn't the end of the world when he doesn't get a full out play session with every dog he meets.
Hope this helps a little!
Leah