sit to earn the opportunity to greet, or there may be other rules attached (such as “don’t pet her if she jumps”). or, there may be other important instructions from the owner (such as “please let her approach you – it might take her a moment or two”, or “please pet her under the chin, not over the head”).
for example, if you see mel uptown with a board-and-train dog, she will likely say, “yes, but… he has to sit first, and be released to go say hi”. we have noticed that most people only hear the “yes”, and as she is trying to get the dog to sit, the person 