Breaking up is hard to do, but pouring your heart out on paper is rather awkward. There is really no need for any other information in your resignation letter (including a reason __ which is completely optional). No effusive template and formatting effort is required. This is not a letter that is going to be hanging on anyone's wall, and it probably won't even be filed away!
The Thank You. Next, it's always a good idea to thank your employer for the opportunity, describing some of the key things you've enjoyed and learned on the job. And yes, this is true even if you're thrilled to be leaving. Remember__you may need these people for a reference down the line, and leaving things on a good note will leave a lasting (positive) impression.