Why write a letter?

I have been writing letters to myself my whole life. I am pretty sure I wrote one to myself at age 8 to be opened when I turned 16, and somehow the letter did not get lost in that time span. I write letters sporadically and then seal them in an envelope that says "DO NOT OPEN UNTIL..." and the date is typically a significant point in my life, like graduating from highschool, college, etc. Usually the to-be-opened date is a handful of years away- I have yet to write a letter to myself at age 50, and don't plan to- that would be really abstract.

Opening the letters is always a fascinating experience. I often ask myself questions- "did you stay friends with so and so?" "did you travel to China?", and the type of questions I ask ("did you date such and such person?") tell me a lot about what was important to me at that time. I sometimes make predictions, or list my wishes for the future. Then I write a little bit about what is happening in my life at the time. How I am feeling, what I am excited or annoyed about. Those few sentences bring me back to the state of mind and the maturity level I was when I wrote the letter, which oftentimes is the most valuable element of the letter. How have I changed since then? In what ways am I the same?

So why am I asking other people to write a letter? I think it would be great to encourage other people to have this kind of experience as well. What could we all realize about ourselves if we all read a letter written by a three-years-younger self? A letter (especially handwritten) is a good format for this- an email will always be saved somewhere and can be read at anytime.

I'd like people to send me their letters so that I can safeguard them and then send them back. I'm not going to read them- I don't want to invade anyone's privacy in any way. What is interesting to me is a huge pile of sealed letters; they are mysterious and private and represent a bit of self-reflection or at least some imaginative thinking sent to me by unknown people.