Aloha! New with a big question!

[brewer]

Guys, completely unrelated but while I got you dog people, I want to ask a dog psychology question.

Around the corner from home, the council segregated part of the parklands into a doggy specific park play area.

It's so popular! Anyway, the wife was late getting her haircut, so I got some subway and parked under a tree there and just watched all the doggies having their fun. There would have been 20 of them, all shapes and sizes, all wagging their tales and running and playing. Except one. It happens to be one of my favourites. It was a bull terrier.
He was walking around slowly and completely oblivious to all other dogs. No eye contact with them at all.
Like they weren't even there. Like he/she was just staring into space the whole 20 minutes I was there.
I actually took some photos but they didn't come out too well.

What do you think is going on? Is it the breed? Only child? No social skills? Naturally unsociable?
I actually felt sorry for it. It just seemed so lonely. But it wasn't making the slightest effort to fit in.
Was never interested in chasing any balls or interacting whatsoever!

I never eben saw the owner approach it in all the time I was there. It couldn't be a stray because the area is gated.

Another thought is medical, especially if older (ya, like no one knew I'd go there). If over weight they are prone to hip dysplasia and patellar luxation. If in pain it may purposely avoid other dogs and any extra activity. They are also prone to deafness (especially the whites). Plus, any/many older dogs can get cataracts or PRA, so maybe blind. No wonder they will isolate. Last, the breed is prone to cardiovascular disease. It may be physically unable to interact.

Dogs that are not physically fit are often "stoic". They don't want to be perceived as the unfit one in the herd opening them up to attack.

Edit: If you're taking your dogs to public dog parks or dog events make sure they are current on their vaccinations. Even then they are a greater risk to catch communicable disease (i.e. kennel cough).