Passion.
Why write if you don’t bring a passion to the page? Always pack along your passion as you gather up your tools to write. Carry that passion throughout the entire experience of research and creating, and it will magically find it’s way onto the page every single time.
The Illusion 
Sometimes you can’t write with passion because you are forced into a situation or have to write about a topic you really have no interest in whatsoever. If you are writing on your own accord, then I agree you must do it with passion. I personally can’t write with passion when the topic is something like, “Discuss 3 symbols in [Insert Book Here]” and when you have to do it in a 5 paragraph style with a strict format and required amount of information.