Not that weird, to be honest. Most people assume that if you're in a public space like a store with your dog that your dog is friendly. While it's not a very smart thing to just let your dog walk up and greet any dog without asking, if your dog was reactive or aggressive it probably shouldn't be in that environment to begin with. While Home Depot probably has way less dogs than a pet store, you wouldn't go into Petco or PetSmart expecting every dog owner to be 100% dog savvy with complete control over their dog. He was probably just happy to see another dog, I doubt he was smiling because he was a smug bad dog owner.