There are three kinds of people that you find in the public service - Incompetent, Principled, and Power-Hungry. Because that's all the sector can attract. The principled ones quickly become jaded and/or burn out. The incompetent don't have anywhere else to go so become life-long dead weight. The power hungry head straight for the management track and/or politics.

There really aren't other reasons to work in the public sector because it pays like shit, you're swimming in incompetence and power-hungry narcissists, and as a group you are viewed very dimly by virtually everyone.

In the private sector the organization rallies under the ultimate goal of profits. In the public sector the organization rallies under the goal of staying out of the papers. This fundamental difference in motivation explains most of the dynamic differences between the two.