Do you think that because your dog is a dog, he (or she) should enjoy being with other dogs? Dogs, like people, have different personalities and preferences. They don't necessarily like another dog just because it's a dog.
Think about it: do you like people just because they're people? Not really. What if I dragged you along to every neighborhood picnic, to every baby shower, wedding, and graduation celebration and expected you to (1) have fun (it's a party!), and (2) like everyone you meet (they're people!) and (3) want to hang out with those people every time I brought you along (enjoy yourself!).
Do you think it would be fair to expect all that from you just because I think the event is fun, I think you should enjoy it, and because there will be other people there?
Yeah, I didn't think so. Yet so many people think that their dog should enjoy the opportunity to play with another dog. Expecting your dog to appreciate the presence of another dog just because it's another dog is unrealistic and actually unfair to your dog.
Give him some credit: your dog has his own preferences. He may enjoy smaller dogs more than larger dogs (Talos, the Great Dane had a best friend at daycare — a miniature Dachshund), he may enjoy chasing or being chased, and may hate to be body-slammed during play. He may prefer to play independently (Tango, my yellow Lab much prefers to retrieve the tennis ball than to play with another dog). Or he may not really be into playing at all (Lily, my black Lab, prefers the company of people over dogs any day, thankyouverymuch).
Next time you get the urge to take your dog to the dog park, think twice. Does he really enjoy being around other dogs? Or do YOU enjoy taking him around other dogs. Watch your dog's body language, he'll let you know.