People talk about things that worry them, to get reassurance that their actions are socially acceptable.
Say you really like pie, though you know it is bad for your waistline. You start letting yourself get away with eating pie every day.
You are feeling a bit worried about this, and tell your friends. They reply "Oh, we eat pie every day too. It's really bad". Now you feel much relieved: I'm not so bad, in a relative sense, because most others are bad in the same way. But what if, instead, all your friends had said "Oh my god, pie every day!, Thats crazy!", then you might have felt a shock of shame, but then you might have gone home and skipped your pie that night.
There is a tendency to blame the messenger when we don't like the news, but if everyone else is in agreement, we stop blaming them, and are more likely to force ourselves to improve. But, if most people say "It's ok, I do it too", and there is one dissenting voice who says "Pie! That's crazy.", there is a pressure to make the oddball conform. The majority would rather not feel guilty, and one easy way to fix the problem is to tell the goody-goody to please shut up.
People are not fools. We have impulses to do all sorts of things we ideally shouldn't, but we use feedback from others to find out which ones are ok and which ones will get us voted off of the island. It's an innate democratic instinct, which lets us collectively hammer out a flexible and constantly evolving social contract. The mechanism is sound, and very useful in many good ways, but it does have the downside of allowing people to let themselves get fat and broke.