This will probably get long...my apologies. It is impossible for me to give you all the reasons, but I'll try to give a few...
It is very gratifying to see a that you made a positive impact on another person's life. I teach HS and although I teach a specific subject, students take SOL (standards) and AP tests to see how well I helped them learn it, I doubt that textbook knowledge is the main thing they take away.
There are two goals I have for each student every year: to challenge them so that they are better critical thinkers and better writers when they leave my classroom. If they can do these two things, they can then take care of themselves in college or the workplace.
More personally, I also want them to see a person teaching them who loves learning, loves reading, is fair and honest with others, and is willing to help those who are struggling. Although helping others sounds great, it is often years before you know that you really did have a postive impact. Sometimes the students you think you failed are the ones you helped the most. A student will come back years later and say how much they appreciated me putting up with their behavior/attitude/bad grades and not giving up on them. They are proud to let me know how much they have changed and gotten their acts together.
I stay in teaching for other, more selfish reasons.
1. I have contemplated some other careers, but will probably never do it. With teaching I get summers and holidays off. With small children of my own, this is wonderful.
2. I am always learning, which is very cool for a geek like me.
3. Each year, class, day, and child is different. Things are never boring when you are a teacher.
4. I'm pretty darn good at it. When people pay you to come to a job you love to do, life is good. Sure, my paper load is heavy and the pay isn't going to make many people jealous, but there are more important things in my life than money. That said, I do understand when others leave the profession because of pay, especially in this particular area where the cost of living is rather expensive, real estate in particular.
5. I truly believe people have teaching in them or they don't. I have seen several career switchers come into teaching and fail miserably. These are very intelligent people with great educations and job experiences, but sometimes that little spark is just missing.
Hope this gives you some things to think. Good luck on your paper. Are you already teaching or studying to become one?