Dog barking and aggression

In need of advice.

JRT 2 yr old male. When he sees another dog on walks he goes into a frenzy - barking, lunging, jumping around.

He used to do this slightly, but was happy once he said hello.
Now he barks constantly, and has 'gone' for some dogs, as in snapped.
The owner still lets him off lead when they think its secure area. The other day in a field, he saw a dog, hurdled over to it, circled it, barked, then came back to us.

A trainer felt it was fear aggression (which I don't understand as he will run acres just to get to another dog) and has tried a distraction method - running in opposite direction, calling, jumping etc. However this does not work.

Would like to understand WHY he does this. And affective method we can use to stop / cut down.

He is FINE once he knows other dogs. He gets along fine with my dog and another 'doggy friend' he has. He also lives with a 9yr old female JRT.

Have met his mum and sister who are such placid, friendly dogs.

Difficult to know who withough seeing the dog - well infact without being able to get into their heads its impossible to know totally

But yes an animal can behaive like that due to fear - offense is the best defense - if he has found charging and snapping works (owners drag other dogs away from him and so he dosent get attacked - even tho he was the one looking like he started it)

My girl would do the exact same thing if let offlead around onlead dogs

The best method I have found is at a large enough distance that she can be calm reward her for looking at the other dogs, being around them and being aware of them but not reacting

The more time like that then they learn that being quite and relaxed around other dogs is nice and nothing bad happens

also imo having doggy friends and nice experiences is really helpful too - of course being really careful about the dogs picked so there are not bad experiences
Polite well balanced (but a bit mad) male dogs are best for my girl although she has some girl friends too

It takes time tho, and hard work, its difficult because you cant always control when scary dogs are going to come round the corner - but it is possible to make it better

I never bother too much with `why`. If we knew what was going on their heads we`d need to be able to imagine thought processes without words and I can`t do that.
If the owner wants to retrain the dog these books will help:

http://www.amazon.co.uk/Fight-Practi...9511538&sr=8-1

and

http://www.amazon.co.uk/Behavior-Adj...9511570&sr=1-1