There are a zillion online articles and blogs and websites and classes that try to teach how to write a query letter or the cover letter for a submission. But the people who need those most are the ones who don't bother learning. (And likely are not reading this blog, I'm just preaching to the choir.) One self-described "grumpy" agent keeps us entertained with some of the worst lines from query letters.
It's especially fun for other agents and editors to read, because we've all gotten very similar stuff. (Please, please don't tell me about your goat farm, or your personal sexual hangups, or how you got the idea for this story while cleaning up after your puking child.)
It cannot be said enough - make the letter professional, write only about your story and not your personal life. And keep your ego in check.