She is close to 3 years old and just recently started being aggressive with other dogs. She will go up to them to say hello and if they show their teeth or growl the slightest she will freak out. She immediately starts to attack. She never breaks skin or anything but I feel that if I don't nip this in the bud now it will just get worse. She has always been very sweet with other dogs and this behavior has never been an issue. Its more with darker dogs than any others...I don't know if that makes a difference. Please help!