What is it about writing a letter to your future self that makes it so creepy?
It seems like a cool idea, but I'm afraid to do it. It's a little awkward. What do you say? Probably it will be all about your hopes and dreams, and wondering if you've achieved them. But receiving your letter will only remind you what you haven't achieved and how fast your life has slipped by. It's a setup for a moment in your future that will be heartbreaking at worst and bittersweet at best.
Even if you have succeeded beyond your wildest dreams, you will still be sad to be reminded how old you are.
And that's if you're lucky. What if you die? It would be that much more tragically ironic if there were some email out there in the ether, waiting to be delivered in twenty years.
This is all assuming you even still have the same email address. I guess that is a pretty safe bet, because I have Hotmail and Gmail, and either Microsoft or Google will control the entire Internet by then.