distrusting mechanics

As you may know I am a mechanic. I have never really understood the dynamics between mechanic and customer. We seem to have extremes, on one side they look at a mechanic as a know all see all, who at a meer glance can solve all the worlds problems (and their machines :), and on the other they question everything. Maybe they heard a horror story of profound deceit. Maybe they were sucessful changing their oil once and think they should be able to fix anything. The been their done that attitude. Either way it's hard to please these customers....I'll just take solace in the fact their machine runs good as new (and their check cleared the bank!!!)