I don't know why. I've never written one before.
Pick a topic that is dear the heart and have at it.
I think I want to write about contemporary poetry, but all I would have to go off of is my own reading, and I'm a horrible reader. I really have been unable to figure out a solution to resolve this issue--apparently teaching adults to read properly is a non-issue in the US.
If you've never written one before, then you should sue all your English teachers. Every single English class I've ever taken had me write at least one essay.
Virgil, they probably assigned them, I just never did them.