Hi all,  I have a 2 1/2 year old GSD.  He has always got on well with other dogs although I have always though he is quite pushy.  When he sees a dog he will lie flat and when it gets closer he will run up to it.  I always ask if it is ok for him to play as he can play quite rough. Grabbing at collars and head in mouth, but until recently no aggression, just rough and tumble play.
I am now finding that he may be developing a problem with especialy male dogs.  (he was neutered at just under a year old).  He plays lovely with a collie we meet, but the last couple of times I have met this dog, after a great time playing, as soon as anyone mentions leaving, he will go for the other dog.  No damage done (thank goodness) but obviously scared me and of course the other owner. He used to greet other dogs lovely on the lead, but again seems to be getting aggresive on the lead now.  2 nights ago out walking with a very submissive Flat coated retreiver, having a great time and we met a young lab (about 18 months).  My lad and the flat coat went to say hello and the lab seemed unsure, going down and very still.  After a sniff around my lad and the flat coat moved off.  The lab seemed to want to start playing but nothing boisterous and my lad went for him  No damage. Over last couple of weeks has gone for 2 other male dogs.
Last night out with my friend and her female GSD who he has known since about 14 weeks old, same age.  The female ran off into some water and when she came out, came running forward and my lad ran towards her and next thing we knew they were fighting.  Usually with any female dog they will tell him off if he is being a pest etc, and he will just bounce out of the way but never gets agressive.
No idea who started that one or why. Any ideas would be greatly appreciated.
Sorry it's so long.

Thanks
Taro

At 2.5 years old he sees himself as an adult now and is taking on this role against other adolescent males, asserting himself as the elder and more Alpha dog, some males don't feel the need to show their supremacy, some do, unfortunately yours is one that does, it is not something anyone can predict.

He is probably going to be quite dodgy around adolescent males for a while, if you don't trust him then lead him on the arrival of another dog.

after a great time playing, as soon as anyone mentions leaving, he will go for the other dog.

This one is easy to solve, don't tell him, just cruise up to him and clip on his lead and walk him away, that way there is no scene and no last minute aggression.

The female ran off into some water and when she came out, came running forward and my lad ran towards her and next thing we knew they were fighting. 

When dogs are wet they have a natural adrenaline rush, this enables them to run around like crazy to dry off, if you have more than one dog you will find that they will run, jump and body tackle one another often with a lot of snarling and noise. However, this generally is not really a physical fight for real, but play which certainly sounds real but isn't.  I'm sure that it sounded quite frightening, but hopefully you will have seen no physical bites or wounds, just all teeth and noise. So a pretty normal behaviour for recently wet dogs.

My brothers two GSD's used to come out of the water like this then walk home best of friends again, I personally would not worry, but you can always stop the behaviour by grabbing your GSD the moment he comes out of the water and leading him before the rush of adrenaline, pulling his lead in so as to avoid any chance of a run and to keep him calm, downside you will have a dog shaking itself all over you for a good few minutes. :-)

If your dog was not wet, only the female, then this is also easy to explain, as far as he is concerned she did not act accordingly when she came running out - not her fault as it was due to her being wet but he saw it as an act of aggression or disrespect.

All in all he has a bad case of his own ego, as do many males, neutered or not, it is the time to be firm with him, and to be on alert to protect younger males, reign him in so to speak when on walks, lead immediately when you see a young male or keep him on a long line, he is not to be trusted right now and no dog owner wants their dog to be hurt.

If his behaviour does get worse, then a behaviourist is needed, above all else don't let him get the chance to get into a real fight and bite for real, there will be no stopping his ego if that is allowed.

I think with some neutered males they actually feel more need to assert themselves if faced with perceived disrespect from adolescents, as he has had the normal male signals removed.

This is a classic case of the dog not being happy with his (reduced) status through neutering, which has of course it has removed the sexual competition element to potential, aggression, but not the hierarchy aspect.

The best Alpha in a group of dogs owned by a Friend was a Castrated male BSD, and second in command over several other males was a GSD neuter.

Thaks for the replys.   I will keep a close eye on him for  while.  I do keep an eye out anyway for other dogs that I don't know to ask if it's ok before letting him run up to play. Unfortunately I think that your right and he is flexing his muscles so to speak!.
Thanks again.