Can only echo what Ben's Mum has said...
It really used to bother me (especially as this came immediately after she had been guarding us, which obviously
was unacceptable!), but I have learned so much more about canine body language and realised that, most of the time, the other dog was actually at fault. I have had to relax and just accept that my dog isn't highly tolerant - but she is fair. She recently met a very bouncy, excited dog, who was just settling into her new home after 5 months in rescue kennels.... but despite her enthusiasm she was
very good at communicating with other dogs and Honey launched straight into play with her. However this morning, a very pushy Golden Lab approached her head on, and headed straight to sniff her bum, with no appeasing body language... Honey immediately woofed a reprimand. The dog listened and backed off a little and they greeted each other with a little more respect.
What I have learned to do is to try and read Honey's body language more and call her out of situations she is looking uncomfortable with before she feels the need to growl and woof. She's getting more confidence now, but at one time it seemed like she felt she had to stay in uncomfortable situations, and she would get more and more uncomfortable until she reacted... by calling her out of those situations (and other ones which I learned would build to a reaction) I helped her to gain confidence in her interactions with other dogs.
Also if I see she is initially unsure of a fairly confident (and possibly a bit pushy) dog, I try to greet the dog myself first - this gives her the chance to check out the dog without its focus being on her....
Ben's Mum makes a really good point - the more anxious you are about his behaviour with other dogs, the more anxious he will become around other dogs, as he will sense your unease and think there is something to be worried about...