Substitutes

I know substitute teachers are a last minute thing at times, also just a place holder for attendance and an authority figure. My question is; Why do people continue substituting  if they do not like the kids they 'Teach'. I am of course referencing that awful substitute which, I can safely say that we all agree that she was not a prime candidate sub for any class, but why does she do it? For instance she was a substitute for French class and she got extremely upset with the kids and called them horrible, why would a person continue to sub for children she doesn't respect or treat with respect? I can see if there was a need to control the children who are "miscreants" and "bad eggs" but seriously I feel teaching should be left to those who love it, and are good at what they do. Okay I can continue ranting on her poor behavior and lacking of proper communication skills with students, but overall, I am just overall perplexed on the matter of how she became a substitute let alone a teacher. Maybe in years past she was a better person/teacher but I am glad that the school has precautionary measures that can be placed if many complaints come in about a certain sub.

Overall I would just like to ask two things. One; Who is your favorite teacher in your life? Two; Who should be eligible to be a teacher/substitute? The second question must be taken with a grain of salt because I know that substitutes are always in demand and there needs to be numerous subs for the entire school system in case of meetings and such.