I have two Labrador Retrievers, and I went to Lab forums but there is very little on them about dog-on-dog aggression, but here I have found very helpful information.
My two are brother and sister and are now app. 1 year and five months old. They have bonded wonderfully with one another and
with me (they have not bonded with one another while not bonding to me, perhaps because I work from home and literally almost never left them at all for the first 10 months we were together after I adopted them at 2 months). The sister used to be overly submissive to other dogs but has developed more confidence and is a sociable, playful, wonderful dog whom I have not trained exceptionally well, but she has been off-leash her whole life and follows me everywhere, etc. and does more than well enough for my purposes.
The brother was more independent and always played happily with all kinds of dogs until I had him castrated at 8 months. Immediately afterwards, he became unpredicatably aggressive towards dogs who stood no chance against him (docile dogs, puppies). He never hurt another dog, but it was a frightening, appalling spectacle. It may have been the ACE tranquilizer that our vet prescribed, but from something dramatic ocurred, or perhaps it was in him anyway and simply came out after the stress of the surgery and weeklong confinement. I put him through rigourous de-sensitiziation, counter-conditioning, etc. and we went daily for off-leash runs at dawn in the swamps to avoid other dogs. He seemed to get much, much better. Finally we went to remote southern Baja for about one month, and it was there that he really seemed to become a reliably friendly, peaceful dog. He interacted on chance occasions with other dogs and was completely fine, even one time after a GSD bullied him for a good while until I was able to get him back in our car. We returned home, and all summer long he was fine. Every now and then he might growl at a dog, but he would stop as soon as I said the word, and we would just move along. But during the past month or so matters have turned for the worse. He does not look for dogs to fight. He prefers to ignore other dogs and hunt for lizards, squirrels, and even cats (he will swim across a river to bark at and chase a colony of feral cats but never comes close to catching any of them). But if another dog approaches him, he can become suddenly hostile and might lunge to start a fight. He will stop the instant I say so, and so we have avoided any scuffles, but it places me constantly on edge. He is a big, powerful lab, and we live in a busy, dog-friendly city where there are always other dogs even when we do our best to avoid them (we go to a swamp at off hours to avoid them so far as possible right now). We will resume traning and a growly dog class this week. I am considering selling our home and moving to the country, where he seems to mellow out a lot. I love my two like my own children and will do what it takes, but all of this has taken the fun and joy out of my life much of the time, as I am stressed out. I do not want them to have bad lives, I do not want him to hurt or even frighten another innocent dog or his owner, and I do not want to be stressed out.