Please someone give me a suggestion 
We have two male dogs, they have both been very well socialised as puppies and loved playing with other dogs in the park on walks.
Over the past year one of them has become very wary of other dogs and occasionally snapping at them, this is a recent thing (he is 3 years). I can't see anythig that could have triggered this behaviour.
Tonight he bit my other dog, he got his ear... There was a fair bit of blood.
I don't have a clue where this aggression came from
he used to be such a friendly dog.
Does anyone have any training suggestions to stop this behaviour?