Dog question

Why is that dogs like some dogs over others. What is the reason for them sniffing for a few minutes and then when you think they are cool with each other the snarling starts? This happened on our walk this morning, Kai saw a dog they both looked they wanted to play then all of a sudden Kai snaps at the dogs nose. Kai has been to the dog park, boarding with other dogs and when the dogs get too nosy with her butt she raises her hackles but I have never seen her snap. Luckily, I am quick with the leash, but I am not sure what happened to cause the decision to snap.