Some wise old man once told me "we want more because we have more"... actually, no one told me that, I read it somewhere, but to explain things: everyone has a certain void in them, a missing piece of shorts, but not everyone is even concious of that. In any case, the easy way of "filling" that void (albeit temporally) is by having things, because obtaining something does give a sense of acomplishment, even if a second later we realize how stupid it was, because we wanted "to get something" but not that something. Its easy, and society gets used to that.