What is the Big Deal about a Little Sin Anyways?

Why do some people (especially religious people) make such a big deal about sin? We all do it. We all have told that little white lie when asked - “Does this shirt look good on me?” Or, “Do these pants make me look fat?” What about when we find that $10 bill on the floor. We don’t ask those around us if they dropped it. We know if we did, they would all say yes. After all, I thought it was finders keepers - losers weepers.