Dogs on leashes are more likely to start fights than a dog who is off leash so leashes aren't always the best thing to have on.
Observe the dogs first before going in. Look to see if there are any dogs acting extra dominant, (stiff shoulders, very high raised tail wagging very fast, almost vibrating, ears high, trying to stand tall and look down at the other dog"
These are signs to watch out for, and there is no reason you can't correct someone else's dog, or your own dog for these behaviors. You can clap, or make a loud noise to snap the dog out of it.
I only bring my dog to the dog park when there are low numbers of dogs and everyone looks friendly. I let my dog say hi through the fence first and make sure all the dogs get along.
After all, dogs can be like people sometimes. They don't have to like every single dog they come in contact with, just like people don't tend to like every single person they meet. But most dogs are very social like a lot of people.
Edit: Hackles raised on a dog is not always a sign of aggression. My dog who has very short fur, has her hackles raised from shoulders to tail when she goes to the dog park because of excitement, and is always very friendly and playful.