One thing I have heard a number of times and in my world it is certainly true "Dog trainers rarely let their on lead dogs say hello to other dogs be they on lead or off lead" This is because we know what could happen. Leads and collars and harnesses are restrictive, they make body language hard to read for other dogs, they also create frustration and and sometimes fear in dogs who either are so excited they're pulling to get to something or they're scared because they have not route of escape should something scary happen to them.
Dogs greeting other dogs should not look like this face to face greeting with frustration pulling can be intimidating and this can cause a reactive response.
if you watch social dogs greeting each other off lead they tend to give each other a bit of space they move in towards each other nose to tail and often circle while sniffing each other.
So why do dogs do this? Well simply they can get a lot of information about each other from the scent of the dog where it is at it's strongest in the genital and anal region. They learn the sex of the dog and it's sexual state, is it entire or neutered? They also learn it's physical state, is it ill or healthy?
A polite greeting look like this, it give the dogs space while they learn everything they need to know about each other, but it also means they are more able to get away should something happen to startle them.
There is a rule that I like to follow with dogs when on walks to encourage polite greeting and play but also to teach the puppy that it can't always say hello and play with every dogs it's call "the rule of 3" and if you've read my post on socialisation you will have seen me mention it before. It consists of three rules that you as the owner implement on walks while you also interact and play with your dog yourself.
Basically the puppy can to do each of the rules when you ask it and this applies to greeting humans as well as other dogs on walks
Rule 1. You must stay with me
Rule 2. You can say hello
Rule 3. You can play with
This rule will give you puppy freedom and let then socialise in a structured way where they aren't putting themselves in any kind of danger
Clair Litster-Huckle has a BSc (Hons) in Animal Behaviour and Welfare and has studied Canine psychology and Canine diet and nutrition.