It's lovely to meet other dogs while walking your dog in the great outdoors. But dogs should learn how to meet & greet politely, without lunging at other dogs her people and not to be right in the face of each other.
Many times the over excitement can be due to a lack of experience from the dog, lack of proper socialisation skills and so forth...
Dogs shouldn't play while on leash with each other, otherwise it'd encourage them every time they see another dog on the street to want to play with them and it can quickly become a very bad habit.
And remember that not all dogs been walked out there are friendly and you don't want your dog to have bad experiences with other dogs or running the risk of been attacked by another dog on leash.
Meeting and greeting other dogs on leash while on 'walkies' should be calm and polite and owner should guide the interactions.
You should ask to the other dog's owner first if it's ok for your dog to say 'Hi' to his dog or not.
Dog Leash Reactivity can happen if a dog got attacked by another dog recently or in a distant past and may have suffered some traumatic experience.
Displays of fear/ aggression can occur, barking at the other dog, lunging on the lead harshly and on some occasions the dog can redirect aggression towards his own owner, when it can't get to the other dog.