I think you are failing to appreciate the way advertising has damaged purely social interaction. In today's world, a huge percentage of the communications from strangers that people experience are attempts to solicit from or sell to them. From billboards to panhandlers to kids selling magazine subscriptions to the ads in magazines to tv commercials.
After a while, you just get sick of being "sold" all the time. It's like, "Just leave me the fuck alone, I don't want to give you my shit, I don't want to buy your shit." Have you ever had a stranger start talking to you and you start to feel pretty cool because this person was drawn to socialize with you, and aftera few minutes they turn it into some kind of solicitation and you like, "Goddammit, I should have known..."
Anyway, I think people hate the fundamental dishonesty and power-control aspect of sale-related communications and are just fucking sick of it, and have learned through experience that if some one starts talking to you for no reason, they probably want something from you. So these people learn to shut it down at the outset.
It's just one insidious aspect of the rampant commercialism in modern society.
~psychoblast~