Cover letters are a narrative complement to your resume.
Why you?
Focus their attention on your most relevant and compelling qualifications. What value will you bring to the organization? (Not what you want or what you can gain.)
Why them?
Be sure to include a sentence to a full paragraph on why you want to work with this particular organization. Talk about them.
Make a Connection
Share a story that shows you connect with their brand or their culture
Tell them how you came to learn that they were special amongst their competitors
Demonstrate why you value them and the work they do