The key to your dog interacting with other dogs, or socialisation, is familiarity and experience. You should expose your dog to as many positive experiences, with as many different people, places and animals as you can.
Is your dog calm, confident and controllable when he is interacting with other dogs or people? Some dogs display curious personality quirks. They're afraid of people wearing gloves or men with beards. Some bark ferociously when they see another dog or are terrified to get into a car.
Socialisation aims to correct this behaviour in dogs. It's a process that should begin when a dog is young but continue throughout adult life. Even a dog that has been adopted as an adult can benefit from socialisation training.
Familiarity breeds success
The key to socialisation is familiarity and experience. You want to expose your dog to as many positive experiences, with as many different people, places and animals as you can.
Simple repeated exposure is not enough, the experience your dog has while in these new situations must be positive. So, inviting a friend's children over to let your dog get used to them won't be helpful if they pinch and poke him or make him frightened.
Catch them young
When your dog is still young, training school and puppy parties in your home and vet clinic are a fantastic opportunity for your dog to become friends with other dogs. Resist the temptation to coddle nervous dogs or reprimand overly rowdy ones. Dogs are social animals and need to learn about interacting with each other, not surprisingly, the best way to do this is through interacting!
Let them meet people of all shapes, sizes and ages and have some treats ready to help reward good behaviour. Remember to never leave your dog alone with toddlers or infants. Very young children might hurt or frighten him without meaning to and if your dog is startled or feels uncomfortable it might turn on them.
Change the route of daily walks and take your dog places he's unused to. The more he experiences, the more comfortable he will be in out of the ordinary situations.
You are the leader
Dog's instinctually have a strong sense of hierarchy and you should always be at the top. He needs to know you are confident and unafraid before he can be unafraid.
If you're trying to make your dog at ease around larger animals but you yourself are nervous, then stop. You'll do more harm than good until you can demonstrate you're comfortable with the situation.
A simple but effective way to help him become socialised is to keep his attention on you while he is in a situation he is unsure of. Your dog recognises you as the leader of the pack because you're strong and competent and will take comfort from the fact that the leader will take care of things.
If your dog is displaying aggressive behaviour, don't hesitate to look for professional help. An aggressive reaction isn't necessarily the sign of a bad dog but it needs to be addressed quickly. Talk to your vet, who will be able to recommend a behaviour specialist.