sudden aggression towards other dogs.

[Blueberry]

Blue has always been very friendly with people and other dogs of all shapes and sizes, but twice in the last week he has suddenly started snarling, growling and barking at other dogs - on both occasions they were elderly dogs.  On the first occasion, a large elderly Retriever was tied up outside our village shop, looking very forlorn and waiting for their owner.  This dog showed absolutely no interest in Blue, but for some unknown reason, Blue took a dislike to him and refused to walk past him.  So as we crossed the road to avoid passing the dog, Blue turned round and started the snarling etc.  He would not be stopped, and I was mortified.  I practically had to drag him away round a corner before he would stop.
This morning, towards the end of a long walk, we happened upon a local chap with an older Jack Russell.  The owner let me know that his wasn't tolerant of young, boisterous dogs, so no problem, he held his back and we passed by without any incident, but a bit further up the road we stopped to chat with a friend who often walks with us, and her Shi Tzu, Charlie.  Seemingly Charlie and the JR are mates, so as the man approached Charlie and JRT greeted one another and Blue went mad!  I presume he was jealous?
Not really sure how to deal with him when he behaves like this.  He is just a year old and entire.  He is 'adolescent' in every sense and doesn't listen much to me at the moment.  It seems a drastic measure, but would castration help or not?

[bizzylizzy]

I can imagine exactly how you felt, - it was the same for me the first time Humphrey showed he'd got a snarl and teeth, just couldn't believe it and was devastated that my sweet little tail wagging clown could be seemingly so ferocious and it actually scared me!
He's now nearly 2 though and he's hasn't turned into a monster, so I don't think its a indication that he's suddenly become unsociable. There could have been lots of reasons for his reaction (especially in the case of the JR as you say he wasn't good with younger dogs). I think a lot could be hormones/age etc but dogs have a way of communicationg that we don't always recognize, it could be the older dog also sent off some message that spooked Blue.
My gut instinct was not to reprimand, but just get my dog as far away fron the situation as possible and try to keep calm (note "try" )! With regard to whether castration would help, - I think the general concensus seems to be that no one seems to really agree!! I certainly wouldn't rush into anything, - (Humphrey's still sitting with legs crossed awaiting my decision!  ).
You've been doing so well with Blue's training, I would suggest just persevering, he will start to return his attention to you as he gets a bit older and in another couple of weeks, the situation might look completely different. I've noticed a huge difference in Humphrey over the last few months, still as mad as a hatter but generally more biddable, - I do think it has a lot to do with age!
 Can't give any other advice but hope it helps!!