My opinion of people deteriorated almost entirely when I first started working. Worked at a Wendy's when I was 16 and can safely say that my view of people in general definitely took a dip. Dealing with the public and their attempts to do something as trivial as order the correct size combo without getting frustrated and upset made me realize the sheer incompetence of the average person on a whole.
There was my sub-manager who would have dope delivered to the back of the store, or the various individuals who'd hand out probably 30-40$ of free food once the general manager left, or the illegal alien grill-guy who would steal bacon sheets from the freezer yet never be fired due to having a near-slave mentality to which the managers gladly utilized in various ways.
I'd say the biggest issue I have with people at work now and in the past is that most people cannot resist injecting emotion into a situation where it is not required, creating drama where drama is not needed. They let their petty internal frustrations manifest into little drama episodes which create whole hosts of new problems which otherwise would have never existed.