Why do other dogs bite our dog

My dog is a one year old 100 lb male british bulldog. He has not spent lots of time with other dogs, mainly with humans. He is very close to all the family members and a good guard dog and great friend to us all. Sometimes we play rough and tumble with him. He has no bad habits. He is however very "in your face" with other dogs when we take him for walks - he goes right up to them and stands rather rigid with his tail up (although all he wants is to play with them - he is a very friendly boy). I think other dogs are threatened by this however because he has now been bitten (lightly) at least six times by different dogs. Fortunately he has never been aggressive back.

Any ideas (apart from the obvious of just keeping trying socialising him with other dogs by finding ones he can play with)? btw we are reasonably experienced with dogs.

Your dog is being INCREDIBLY disrespectful to the other dogs. I can't even begin to tell you how disrespectful your dog's greet is towards the other dogs. That is like someone slapping you on the face when you say hello. That is how bad your dog's behavior is. He may not be aggressive, but he is rigid and has his tail up - obvious signs of tension and being unbalanced. The other dogs that bite him are doing what comes naturally. That negativity being displayed by your dog is extremely unwanted by all other dogs. I do know this - if your dog was to greet my dog that way, my dog would pummel his butt, and my dog is a generally laid back girl. She HATES dogs approaching her with that attitude!