The passerby that asks you to take their picture is becoming less common. Although almost everyone says "yes" if someone asks, "Hey, want me to take your picture together," fewer people will ask someone to take their photo first. This may have to do with the complexity of modern cameras (almost everyone asks, "So which button do I push?"). It might have to do with people rushing here and there, and a hurried walk is the best way to avoid anyone asking for a favor. But more than likely, given today's society where neighbors seldom speak over a fence and many simply shut their garage doors using the remote control the moment the rear bumper passes inside the safety sensor, it's just a matter of not wanting to interact with another human being.