It's fear..and/or frustration.
There is LOTS you can do to improve this and contrary to Dogsausages assumption, he is very unlikely to be a dangerous dog..he's a nervous one.
You do want to find a trainer but you need a postitive reinforcement only trainer (not the 'dominance theory' type.. that myth has been debunked years ago)
Meantime.. join ReactiveDogsUK on fb...
www.facebook.com/groups/605603546664098
it's a fantastic group run by people who know what they are doing, and also a huge source of support..and there are files detailing exactly HOW to help your dog... however a trainer is really useful because the timing of positive reinforcement is crucial.. and you aren't using treats to distract your dog, or sooth his.. the CARE aproach actually teaches your dog to associate scary thing (other dogs) with the best treat ever..and gradually it actually changes their emotions towards other dogs. (This also works with anything else they are reactive towards.. my dog is selectively dog reactive and too full on when playing, and also hates vets, and anyone in a black hoody.. long list!)
Grab a cuppa.. and read this first.
[http://careforreactivedogs.com/start-here/]]
It's not a quick fix.. (sure you can yank your dog, make them sit etc but that only supresses it.. ) but it CHANGES how your dog feels and trust me when it starts to work it is amazing. You may find you have to be a long way away from other dogs at first.. the distance where your dog is TOTALLY unbothered by the sight of another dog is where you start..and that will a lot further than you may suppose!
When I see another dog and we are too close for my girl to be totally relaxed I watch her body language.. the second I see her spot the dog (not tense, or pull.. just LOOK) I mark it with YES!!! and shovel high value treats til the other dog is out of sight. The second they have gone the treats stop.
See a dog? YES!!! Duck rains down on me (her fave) Dog gone?! Cu[board closed. Now if she sees another dog she tutns to me for her treat.. and we can pass dogs WAY closer than a few months ago.
It works. And it's kind, it's bonding and it takes a LOT of stress out of our relationship!
Sorry this is so long, but I am passionate about positive training for nervous (and bolshy playing dogs are generally always nervous)
Have a read, find a good trainer... (accredited force free)
Oh and it's OK not to walk your dog.. if he's had some bad reactions.. give him a day off, let his cortisol levels settle.. it really helps!