A note on the cultural portion.  Part of our culture is to be bombarded by advertisements all day long that you should eat this or that. When you are shown every few minutes that happiness comes from eating a full McDonald's meal, the half of the population that is below average in intelligence and therefore above average in gullibility tends to want a piece of that happiness. In a world of ever increasing depression, it's easy to buy a few minutes of happiness while not thinking about the guilt/shame that follows.