There's a lot I could say about the discouraging factors of teaching, but instead of going on and on about those factors--factors most teachers experience, I'll instead focus on the joy and happiness children bring to life.
The reason so many of us teach is because we enjoy working with children, and we all know there is wonderful magic that happens in that sphere.
So if you're ever discouraged, make time to spend time with children in positive endeavor. It will definitely bring a smile to your face.