I don't know if it's just instinct, but I never force petting on a strange dog. It's just common sense, particularly when even a small dog can administer a nasty bite, and one never knows what kind of mental baggage a strange dog may be carrying. (Or even a known one, for that matter ... my neighbor got bitten right through his eyebrow by the Cairn terrier he had owned for the past three years, that had never shown any biting inclination.) In a way it's similar to shaking a person's hand. You make an offer of your own hand, and allow the other person to accept that offering on his own terms. One doesn't come up on someone else and simply seize his hand, willy-nilly!