Diversity makes teaching fun.
As a history teacher, I want my students to learn how to argue (form opinions, derive evidence from primary and secondary sources, and apply that evidence towards proving their thesis). Arguments and debates work the best when there is a difference in perspective and background–in short, when there is a disagreement worth arguing about. There is nothing wrong with having a classroom full of students who all have similar ancestry, religious beliefs, and political beliefs; a good teacher can generate debate even then. But it is easier to inspire debate when students approach the material from very different backgrounds. Each student has their own challenges and opportunities, created by their ethnicities, their economic situations, their family situations, and their own physical, emotional, or psychological differences. My job as a teacher is to harness these differences to the benefit of all.
Doing so requires that the teacher set up a classroom culture where ideas can be debated without impugning the character of the people advocating those ideas. It requires that both the teacher and the students be willing to treat each other with courtesy and respect, to be willing to ask for and offer forgiveness, and be willing to approach the material–and each other–with an open mind. The teacher needs to know how to push each student to improve themselves, while allowing students to go at different paces and perhaps even to approach the material from different ways. Harnessing these differences effectively is certainly challenging, but that is exactly why it is so enjoyable.