The purpose of a cover letter is to just bring up things that aren't in your resume. If you have nothing to say, make it as brief as possible, maybe even one paragraph. Most employers will open the resume anyway.
Don't try to fake passion, that just makes you sound like a teenager desperate to get laid.
My format:
First paragraph, say what you are applying for if it's not in the title header.
Second paragraph, tell them how you meet the requirements. This is where you make it clear you have read the job ad and aren't resume blasting.
Third, explain other unique skillsets you bring. I like to emphasize my entrepreneurship and product development experience or that I can do full stack, if applicable.
Fourth, why you want to work for this company specifically.
Each paragraph would ideally be 1-2 sentences. The shorter the better. If not applicable, don't write it.