Why is my dog being aggressive?

My dog is a pretty friendly guy. We go to the dog park to socialize and get lots of exercise walking and running together. He tends to get aggressive towards other dogs only when we're out on a walk. I don't really understand what's going on. He's totally fine in the dog park and runs around nicely with the other dogs, but if he's on a walk he goes nuts (ie pulling and barking and making a fuss in general). Any help or advice would be much appreciated!