This is a really good question! I'm a retired middle school art teacher, and her is my short answer:
The best part of teaching is being able to work with kids and share your enthusiasm and knowledge about your subject. Nothing is better than seeing your students get excited about learning. The worst part is dealing with the behavior issues of some of those kids; please understand it is a challenge that you can learn to overcome with time and experience, so don't be put off from teaching.
I hope this helps.
Working with kids is an extremely fulfilling career and brings so much joy to your life. It's great to be able to have such a huge impact on the lives of the kids and being able to share your knowledge with them. I think that is definitely the best part of being a teacher. The worst part, like Ellen mentioned, can be the behavior issues. It is frustrating in the beginning, but it gets better!! Teaching is one of the most important jobs and its extremely impactful!