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!
Why a Resignation Letter? In these digital times, why should we submit a resignation letter, anyway? A paper trail does seem old_fashioned, but in this case, it's a best practice. It makes your intentions clear, as well as the conditions. You're removing any ambiguity about the end of your time there.