Can't we all just get along?

" I can't figure it out, my dog used to like other dogs, but now seems to get aggressive when other dogs come near...I'd like her to be friendly..." 

Despite what Disney might portray, not all dogs like other dogs. This doesn't make these dogs, ' bad', instead. these are dogs who are looking for more personal space.

As a puppy, your dog might have been very dog social - actively seeking out and interacting with a variety of dogs. But as she matured, her desire for play with other dogs, familiar or unknown, may have shifted towards tolerant ("You are OK as long as you keep your space") or selectively tolerant ("I like little white fluffy dogs. I don't like big dark colored dogs" ). And sometimes, even with best breeding and carefully orchestrated socialization experiences, your dog develops into a dog Intolerant dog.

 Most adult dogs tend to be Dog Tolerant rather than Dog Social with other dogs - a sort of  "I'd like to hang out with you, but not roughhousing" attitude.  Socially tolerant adult dogs will tolerant rude dogs to a point, then will start a conversation along the lines of... "FYI, I'm giving you a warning"  . A well balanced socially adept  dog will pitch this conversation at the right level to get the meaning across to the other dog. It may be simply a stiffening stare, or a tight closed mouth - or it may involve a flash of teeth, or lunge towards the other dog. The point is, that the intent of the conversation " Stop what your are doing & give me some space." is quickly understood and the socially inept dog learns to moderate his approach or perhaps go off somewhere else . It is a very quick, momentary conversation and then everyone goes happily along about their business.

Troubles arise when a socially inept dog runs into the space of a  Selectively Tolerant or Intolerant dog. The conversation about making space may be very abbreviated - especially for a dog who has learned through experience that some dogs don't take "Go Away" seriously. And it may involve physical contact. This is when you will truly find out how much Bite Inhibition your dog has - Does the conversation result in a Level 1 bite (no contact)  or Level 2 (slight puncture) or something more serious - deeper bite wounds and possible emergency veterinary care? Not to mention the impact of emotional trauma of the event for all parties concerned, human and canine.

As a responsible dog owner, you need to know where your dog's preferences for interactions with other dogs lies.
*  the Socially Tolerant dog - you still  need to supervise the interactions with other dogs as a sort of a playground monitor.. Observe. Allowing good play and social  behaviors with other dogs = continued interaction in the moment. If your dog is beginning too become rude or  over-the-top in play with the other dog, it is time to step in and remove your dog from the conversation for a few moments to calm down. Play can resume a bit later.
* a Selectively Tolerant dog - note which dogs your dog can play/ interact well with, acting in a socially tolerant manner. Approach these dog owners and talk about set up "play dates" in another location which is relatively dog-free. The goal here is not to throw your dog into a variable pool of dog personalities - some of which are OK for your dog's personality - some not. 
*  If you have a dog intolerant dog - sorry - your dog should not be in locations where other dogs are likely to invade his or her space. on OR off-leash.
"But I want him to be friendly with other dogs" " She needs to be off leash & run to exercise" . Living with a dog reactive or aggressive dog means that you need to take special care not to place your dog in situations where he or she is likely to have to use behaviors such as barking, lunging or biting to make space happen. Repeated exposure will NOT make your dog more 'socialized' despite what your friends may offer as advise.

Here is the takeaway from this discussion:
Know how your dog handles the presence of other dogs - whether it is the event of dogs coming into your dog's personal space (on or off leash)  or how your dog reacts when seeing another dog.
Dogs do learn from each other.
*  If you have a dog Intolerant Dog, don't be the agent that shifts a Social or Tolerant dog into the Dog Intolerant league.  Support your dog and his or her preference for not having to be in the company of other dogs.
** If your dog is a happy social butterfly, don't allow him or her to harass a Selectively Tolerant dog and effectively create an Intolerant Dog.   Your overly friendly goofy dog can be just as much of a problem as a small terrier who emotionally blows up when seeing another dog.
"Letting the dogs work it out" is not a particularly safe strategy to follow.

Be a responsible dog owner. Support your dog. Protect  and respect their social tolerance or intolerance towards other dogs.  Your dog does not have to play or interact with other dogs to have a great life.