I took on the responsibility and ownership of a stray Pit approximately 9 or so months old on June 8th of this year. He now may be a year old or soon will be. Of all the many dogs dumped in my vicinity over the last 29 years, I've kept none until now. This one dog is exceptional. He was immediately vetted, neutered and put into the best Dog Obedience School around here. It is my intention to go into advanced dog obedience with him and eventually do Pet Therapy as I did with my Rottie 16 years ago. I keep reading and hearing that somehow he's going to become animal aggressive upon reaching 18 months to 2 years of age. Up to this point, having had him 3 months, he's not so much as growled at anything or anyone but his basketball. Yesterday he snatched a screaming baby rabbit from a snake's mouth, then dropped it upon my command. The snake and the bunny lived! It is hard for me to believe he will one day unexpectedly decide he's a dog killer. Can't you see us heading up the elevator at the Veteran's Hospital one day in the future and all of a sudden, the Pit attacks the other Pet Therapy dogs?
The Pits I have known personally have been like this dog - exuberant, jolly, happy-go-lucky clowns. Please note: I will deal with his aggression, should it all of a sudden develop. I'm just wondering if it is inevitable. In the meantime what, just continue enjoying and working with him while waiting to see? On our walks, he's encoutered barking, loose dogs running at him, and so far he just stands there, waiting for me to run them off. We did have an unneutered, male Pit visit Dog Obedience class the other day. He kept a close eye on him and was somewhat distracted by that particular dog's presence. He has a website that chronicles his progression from stray to family pet. It is http://www.breauxtales.weebly.com if you'd like further information on him.

its not inevitable,but with an unknown parentage may be likely once the dog reaches full maturity, which can take up to 3 years in some lines.
imo, its more a matter of breeding, than breed(well, handling too). considering it was a stray, it likely didn't come from the best of breeders, or the most stable of lines. Even solidly bred fighting dogs(or, at least back in the day, not anymore) were fairly social with other dogs that were younger, or not actively challenging the dog.
That being said, you could have one of those APBT's that live their entire lives as dog social butterflies, and there are many of them, so there is that possibility.
from the various bully breeds that i have known though, that "i love every dog and every dog loves me" attitude will diminish as the dog matures.

Ideally, you want to end up with a dog that is neutral to other dogs, not aggressive towards them, or hellbent on meeting them either.
with a dog that will be working, especially around other dogs, i would think you WOULD want to strive for being neutral, and not having either extreme(overly social, or aggressive with dogs), a dog that gets all wigglebutt and squirmy and distracted when another dog is there is just as frustrating, for me at least, than a dog that is reactive to other dogs.