Why do mechanics always try to screw you over?

I know that people always tell you to never take your car to the dealership to be worked on but I have to say, I love dealing with Saturn provided I get to deal with Ellen. She works at the Saturn of Norwood and she is fantastic. The other guys there can be real jerks but she is great. I actually have it in my service record that she is the only one that can call me about my car. Whenever I go to Saturn she calls me with a quote and they either come in exactly at the estimate or under....usually under. Only twice have I ever taken my car somewhere else. The first time it was a local mechanic here in Upton who raised the estimate three times, charged me twice what Saturn does for the same work, and managed to deprogram my sunroof while he was working on my wheel bearings. Obviously I'm never going back there.

This week I need to get new brakes. We're leaving for Niagara Falls tomorrow and we definitely didn't want to head out before getting them fixed. As much as I love Saturn it is not very conveniently located and I wasn't able to get in for them to fix my brakes before we left so I had to head down to Meineke today. Before I did I called and talked to Ellen. After looking at my service record she said "Do NOT let them sell you new back brakes. You don't need them fixed." The problems with my brakes have been that they've been pulsating in the front whenever I brake. Meineke just called me and said that they think they're pulsating in the back and I need new back ones. I told them "no thanks, just the front ones will be fine" and then he told me that yes, I do need new rotors on the front ones. I know that if I hadn't talked to Ellen I'm sure I would have bought new rear brakes as well. Why do they always try to screw you over? I am going to make sure that both Hannah and Jake take auto shop in high school so that they at least know the very basics of car maintenance.