Got my Job Call and my Employment Gun right here...

So I'm now hunting for a job. I'm astounded by how difficult I find it to write a cover letter. This is a page of text, at most. It just has to say "Hi, you have a job and I need one, here's why I'm your guy." Why do I find it hard to even get past "To Whom it May Concern"? I know I just need to suck it up, and get it done, but it's odd that I have this block.