Most dog walkers are very respectful of others, and keep their dog on a leash whilst out walking. there are many reasons to keep your dog on a leash, the main one being that the dog is firmly under your control when out and about. It is also safer for your dog, as unleashed dogs can get very excited by the sight of cats, their natural instinct is to chase which could result in your dog getting injured, especially if their is traffic nearby.
Unleashed dogs can also be a hazard for small children. Although you may know your dog to have a very pleasant temperament, the sight of a four legged fur ball racing towards them can upset the most stoical of children. Legally dogs (in most areas) should be kept leashed when outside in public areas, if your dog causes any kind of accident whilst off the leash you will be held liable, and this can result in very heavy fines. Even if your dog is attacked by another dog whilst off the leash it would be very hard to claim for any type of compensation.
If you allow your dog to run around whilst off the leash you could be considered as a public nuisance. Dogs are easily excitable and may cause accidents such as knocking a frail person over. They may become to excited to listen to commands, which means that the dog is not under control. Should your dog bite somebody whilst unleashed, then you will be in serious trouble.
Another reason to keep your dog on a leash is so you know exactly what they are doing. Unleashed dogs may eat all sorts of things whilst you are not looking which can cause stomach upsets or small bones to get lodged in their throat. There have been cases of dogs eating poison with disastrous effects.
Keeping a dog on a leash applies to dogs of all sizes and characteristics. You may have a friendly hypoallergenic dog that is very well behaved, but you will still be expected to keep your dog on a leash.