As a dog lover or even an animal lover, in general, the sight of a working dog that appears very friendly can be an almost irresistible trigger for the need to pet it. It is important however to understand that working dogs need to focus on their handler and should not be distracted from their tasks at hand. Working dogs of all types, from police dogs to service dogs often have a handler who either asks you not to pet them or equips their dogs with vests stating clearly no touch, no talk, no eye contact. While this may seem harsh, there are very good reasons for a no touch policy.
Take for example the case of a dog trained to assist his or her partner with mobility issues. These dogs are often trained to brace in place and help individuals keep balance. A dog distracted by petting or attention can inadvertently cause their partner to fall. The same can be said for distracted dogs for those who are blind. Often people assume that when a dog and his handler are sitting down at rest, it is a perfect time to approach and pet the working animal. Once again, the distractions can prove detrimental to the job of the dog or the handler. A dog trained at detecting oncoming seizures, fainting or low blood sugar may neglect this task if engaged in seeking and enjoying attention from other individuals. It is, therefore, best to assume that when a working dog is out in public, it is always working. Unless you are given explicit permission and invitation by the handler or partner of the working dog, you should not attempt to pet or distract the dog in any way.
This certainly does not mean you will never be able to pet a service dog. The most important thing is to ask the handler first, preferably before coming within petting distance and when the dog is not visibly engaged. Under no circumstances should you approach and pet a working dog without permission, especially police dogs as you may find yourself in a precarious situation if the dog reacts unexpectedly. Be sure to respect those who have no touch, no talk, and no eye contact vests on their working dogs and refrain from asking permission. Remember to always consider the safety of a dog's partner before your own personal desire to pet.