How to keep your dog happy on the end of a lead:

While dogs cannot speak, they do display their state of mind with body language. Dogs use complex body language to communicate when meeting, greeting and interacting.  When dogs normally meet (or even pass each other) they usually curve in a circle toward each other and meet head to tail. To walk straight toward another dog is the equivalent of a direct threat in dog language.
When we put a dog on a lead or use any other equipment to direct them we interfere with the dog’s ability to communicate with other dogs and people. This can cause significant, even severe stress to a dog because he can feel out of control, unable to communicate and defend himself. He may also feel unsure that his owner understands, or if his owner will react appropriately. If owners are unaware and ignore their dog’s signals they often break every rule in the canine rule book. This can cause much distress to the dog and he may lose confidence in his owner. This may develop into "on-lead aggression" and "socialisation" problems.
On-lead aggression is the most common type of aggression found in dogs. To owners oblivious to their dog’s body language, this can seem to appear out of nowhere, but in fact is often the accumulating result of the dog being constantly barrelled over, confronted and placed in uncomfortable situations time and time again, until enough is enough. With a little bit of knowledge and practice owners can become aware of their dog’s body language and practice polite behaviour whenever walking their dog on-lead. There are many ways you can help your dog cope in awkward situations and these, to a certain extent, will depend on the dog and the situation.
You need to be aware of your dog’s body language and observant of anything that might be seen as a threat to your dog (eg. another dog walking straight toward you), or even an awkward situation (eg. walking through a doorway with other dogs). Always walk your dog past other dogs and owners at a distance that your dog feels comfortable with (this is where an understanding of body language is important). This could be 40-50 metres away, this is determined by the dog. If you feel that your dog will not be able to walk past another owner and dog without reacting, ask for your dog’s attention and walk in a deliberate curve out and around the other dog and owner. After time you can reduce the distance between you and other dogs until your dog is consistently offering the desired behaviour and happy to follow your lead. It is always best to walk past the other dog and owner with person to person not dog to dog. By curving around anything that concerns the dog, the dog will have much more confidence in his owner and be less stressed or reactive at that time and in the future.