Why people go into public defense

In the course of answering the question, "Is there any justice in the criminal justice system?," Washington neighbor The Judge says:

First some observations. I worked for 11 years as a public defender... These days people typically go into public defense for one of two reasons. Either they have a profound belief in the fundamentals of the constitution, or they have a strong anti-authoritarian streak. Sometimes both. But with some exceptions (as in any profession) PD's are skilled, talented professionals...