The more we try to prove and promote our sincerity, the more insincere our interior monologue becomes, the more our lives are in cosmic bad faith with ourselves. (Fiction, by the way, may have first taught us how to read people this way, judge their sincerity and whether they are being "true to themselves" -- another way the fictional imagination may be harming us.) Soon the questions haunt us even when we don't go out on the street; we live every minute with the sense of our phoniness, and wonder if the people who pretend to love us can see right through us and our posturing. There ends up being no haven, no place where the guard can be let down, where one can stop performing the role of "one's true self." This is one of De Zengotita's main points in Mediated -- culture has made all of us "Method actors."
One of the many ironies about this is that we have become less tolerant of "inauthenticity" in other individuals, but we are more and more indulgent of coroporate interests when the dupe us with silly ads or bad product. We love to suspend disbelief for ads, but we give no quarter to our friends.