my dog wont play

Question
I have a 5 year old pug and he used to be very playful when he was younger. Now he is kind of lazy and sleeps a lot. He has a lot of toys but never uses them anymore. He is not interested in them at all and he won't even sniff them. My dad says he is too old but I don't think 5 is that old and my friends have older dogs (one is even 9) and they are all very playful and really enjoy their toys. All Rocky seems to care about now is mostly food. Why won't he play anymore and how can I get him to be playful again? He usually gets walked each day and seems to like that but we took him to the dog park and he wouldn't play with other dogs. A lot of time he spent sniffing the grass but never played with the other dogs who wanted to play except another pug. How do I get him to be more social with other dogs? I would like him to actually enjoy other dogs and play with them. He doesn't really seem scared of them, he just kind of ignores them but sometimes would hide under my chair.Most of the time he just sniffed the grass and kept coming back to our chairs but I want him to go see the other dogs. Thank you.

Answer
Dog parks aren't for all dogs.  Many dogs don't enjoy meeting and playing with dogs at dog parks.    Sort of like people who don't enjoy going to bars and talking to drunk, obnoxious strangers. :)  

If your dog doesn't like playing with toys, be sure that you are providing the kind of toy he likes.  Does he like non-edible toys he can chew, stuffed toys he can dissect, puzzle toys that deliver food, fetch toys? Show the dog how to play with the toy and play with him.  

Pugs can be couch-potatoes, but if this is a sudden change in behavior, you should discuss it with your vet to rule out any medical issues.