It seems like a great idea, dogs hanging out and playing in a secure, fenced in area while their owners socialize. Except, when it's not. Here are a few of the reasons that I believe dog parks work better for people than dogs.
First, the initial greetings can be intimidating for even the most confident dog as all of the dogs in the park rush up to your dog. Imagine walking in the door at a party and everyone there rushes up to you at once and surrounds you. They are all saying hello, wanting to shake your hand, patting you on the back, the arm, the head until you feel claustrophobic. You don't know if they are all friendly, maybe they are, maybe some are intimidating. All you know is that you're feeling overwhelmed and react by saying, "Hey, give me some space!" Suddenly, you're branded unfriendly or aggressive. That's exactly how your dog feels entering the dog park.
Second, there is nowhere for a scared dog to hide or escape in the dog park. If your dog gets intimidated by another dog, he can't go anywhere to collect himself. He hides behind you and you tell him it's okay. Well, maybe it's not to him.
Third, too many people truly do not understand dog behavior. Or worse yet, many people think they do and really do not, especially with regard to aggression. Some dogs are noisy, growly greeters and players; they are not aggressive dogs, but they can intimidate people and dogs. Other dogs are truly aggressive and their owners don't recognize it and don't discourage the behavior. Dogs can learn a lot of bad behaviors from other dogs at the dog park.
Fourth, take a look at how much exercise your dog is really getting in that dog park. There's some running, then a lot of sitting around, maybe a little more running, but mostly sitting around.
Obviously, this may not apply to all dog park situations, but it's common enough that an honest look is warranted. Dogs definitely need canine companionship and exercise, but from what I've experienced, dog parks aren't the ticket.