I would guess he's just naturally reserved - he's not a touchy-feely dog. But if people insist on petting him when he doesn't really enjoy it, he could become more shy/cautious, then fearful, and possibly even get snappy. If everyone who you met on a walk gave you a quick fondle... well you get the picture!
I would tell everyone that he doesn't like being touched/petted, and ask them not to touch him unless he approaches them. Then they could give him a quick rub on the neck or chest (often dogs hate hands over the head) and leave it at that unless he clearly requests more. It shouldn't be about him letting people pet him, he should want it. You'll probably have to be firm - a lot of people expect all dogs to be like labs and spaniels and have no clue of the doggy body language that shows he's only tolerating being touched.
Oh, I'm just the same! I spend far too long talking dog!
So, today as I was walking Zappa, we encontour a little 6 month cocker spaniel that zappa already met before, and played too. today, zappa seemed to be very playful with him, wagging his tail, but in the air, nose down, the little pup is full of energy, always jumping towards zappa, and suddenly zappa growled a bit (maybe 1 or 2 secs growl). I was surprised but it did not seemed he wanted to hurt the puppy, he continued to be with his tail waggling and jumping etc, the cockerspaniel owner was surprised too, she asked me if he would bite her puppy. Immediatly I took zappa from the "play", and he growled a bit more (once again, 1 or 2 secs). The cocker owner said that he seemed very very playful (I thought the same) and happy.
My question is: Is zappa becoming aggressive with other dogs? a bully with other dogs? or is it just his growling is getting more adult (like a young boy changing his voice) ?? He seemed very normal, but I did not expect the growling part and is starting to worry me... Any ideas?