Hi,
I'm asking for my sister so I only have basic info.
My sisters Lab is approx 18 months un neutered. He is a lovely dog but is starting to become aggresive towards some random dogs while out walking. On and off lead. Usually he loves most dogs he meets and apparenly he has always been submissive.
Now he has started going for some dogs.
One recent instance he was playing in a field with a Golden Retreiver for an hour and they seemed happy. GR kept trying to mount there Lab which he didn't seem to mind. Then a 3rd dog another male came along. GR tried to mount there Lab again and there Lab suddenly went for GR really nastily. Brother in law had to pull lab away. Any ideas whats going on?
Is it his age? Is he becoming dominant? Why can he be on a walk and see another dog just try and go for them?
Only been past couple of weeks.

Sorry if i'm a bit vague.
Also he starting to become territorial of garden gate. Some strangers can walk past, others he seems to dislike and he looks really aggressive!
Any thoughts

The GR's owners should have stopped their dog immediately, it wasn't fair on your sister's dog.  Very few unneutured maturing males are going to allow this sort of behaviour, fisty cuffs were always highly likely to happen.

Calculated guess would be that the random dogs aren't really so random and are entire males.  Just a guess.

Not wise to let a dog repeatedly mount another (unless mating, of course) particularly one entire male to another- it will often end in tears. The fact that your dog endured it for a while without doing anything shows he is not really aggressive. He is now a young man and doing the sort of thing many young men do- a bit of swaggering and a willingness to take on all comers. Many young males will go through a phase of wanting to have a go at other male dogs. My advice is to have him on a long line and watch out for dogs with testicles and avoid or distract him so he doesn't engage. You want him to be able to be around other males but to maintain a respectful distance and thus stay out of trouble.

If you keep an entire male and he is a fairly red-blooded sort then, I am afraid, you will always have to be a bit vigilant. There will be other male dogs he's okay with (once fully mature they simply tend to ignore each other) but there will be others that he cannot stand and regards as rivals for his turf. Once you know which dogs these are I would simply avoid them. Unknown entire males ditto.

Often dogs of similar size, age and weight will sort things out between themselves, but if you are out in the park or countryside you don't really want entire males of different sizes getting into fights. Just watch your boy's body language, if he approaches other males head up, tail up, strutting, possibly with fur on his shoulders and back up, then call him to you and reward well.

I was amused to watch the power of body language with my dog. He and another male were approaching each other head on- the other dog much bigger. As I was just about to call my dog to me the approaching dog's tail and head went right up- instantly my dog turned away. He has learned how to defuse a potentially tricky situation. The smell of the advancing male would have warned him of the other dogs status and so on.

> The fact that your dog endured it for a while without doing anything shows he is not really aggressive.

That was my thought as well, I'd say it was a testament to his good nature in fact!

I think your sister's lab put up with a lot before turning on the GR.
If it was my lab i would have taken  him away from the situation before he got fed up with the GR jumping on his back constantly.
As for his gate..i would say he is just doing his job and growing up to an adult dog looking after his territory.