I am always surprised that people don't 'get' that towering over by a dog 1/2 to 1/10 your size isn't a great way for a dog to feel safe, secure, or comfortable.
I always crouch down on the dogs level to day hello, and if they are shy I just talk to them. usually they warm up enough for a quick scratch, but not always (there are some exuberant pups who don't give me the chance to crouch down, but they usually know me on sight)