The cover letter and resume are what you use to sell yourself. If you are not the right person, you can decide that in the interview process, however you need to get to the interview process first. You need to convince the HR person or whomever is reading your resume and cover letter that they need to search no more. You are competing against hundreds (and sometimes thousands) of others. Why are you so special? You need to convince someone in 30 seconds or less - and it should be specific to the job posting - not just a general cover letter that you use for everything.