Friendly to Some Dogs not Others

Question
My lab mix is very friendly and sociable to some of the neighborhood dogs when we are out walking, but not to others.  Most are met with a wagging tail and sniffing while others get an extreme reaction of snarls and aggression.  Why is this and how can I predict how she will react upon meeting a new dog?

Answer
You really can't.  Dogs pick their own friends.  One of the reasons this happens is that one or the other of the dogs may be sending a negative vibe, and neither dog can get away, due to being on leash.  So, they do the only thing dogs know how to do - they tell the other dog to back off.  One way to avoid this situation is to not allow your dog to meet and greet other dogs at close quarters while on a taut leash.  Many dogs that react the way yours does are fine with other dogs when they are off leash, or if the leashes are dragging, rather than tight.  Read "Feisty Fido" or "Scaredy Dog" and you may get more insight into this behavior.