Why do people suck?

I turned 32 recently. 17 of these 32 years have been spent in the glorious industry known as service.

Working my way up through the ranks I have encountered thousands of guests, hundreds of co-workers and countless managers. In these 17 years I have had some wonderful, enriching jobs as well as some horrendous, nightmarish jobs. It comes down to the leaders. The wonderful, enriching jobs have outstanding leaders while the horrendous, nightmarish jobs have incompetent and petty leaders.

My question is two fold. Why do people suck? What is the difference between the incompetent and petty leaders and the outstanding leaders.