Ditzy Doo,
When you find this letter, you will have been standing on the clouds in Cloudsdale awaiting my return from a mission that I told you no information about and told you you couldn't come with me. Before you continue reading this letter, start flying away from the city as fast as possible: it is very important for you to do this.
If you are reading this letter, I have failed in that mission. Because of this, I am unable to come back to talk to you, for fear I will make an even greater mistake and ruin your planets possibility of a return to its normal way of life. From this point forward, you are a crucial part of the timeline, and as I have mentioned about Twilight Sparkle and some others, I cannot travel with peop- ponies who are critical to the timeline. While I cannot tell you about future events in your timeline, I can talk about what has happened in our past, and just how vital you are to me.
When I came to this dimension, you were the first pony to see me and were the first one to really accept me for my differences. I asked you if you wanted to travel with me, or you asked me if you could travel with me, to be honest, I don't really remember, but either way, you traveled with me for almost a week before I took you back home. As you were leaving my TARDIS, you did something absolutely no other companion I had traveled with had done. You asked me "Same thing next week?" It is impossible for me to impress exactly how much this simple question impacted me that day.
Over the years of the traveling you did with me, you asked me multiple times why I chose to travel with you, regardless of how many times we have had bad receptions in the places we have gone. I am sorry for not truly answering your question any of those times you asked it, simply brushing the question off by answering with, "You see the world differently than me." To finally answer that question: You reminded me of myself during my original life. I was always curious on what everything was, but was somehow able to figure out how something worked, or the theory behind the machine. Before you came into my life, I was getting ready to stop, I was going to give up. I had lost more friends than I care to try to count, and I felt that I could not continue because of the pain. Despite how everyone treats you, how many bad things have happened during your life, you face it with a joy that I found impossible not to share. For that gift you gave me, there is nothing in the world I could give you in exchange.
As much as it pains me, I do not believe I will be able to see you again, as even if I survived my failure of this mission, if I were to see you, I do not believe that I could resist attempting to protect you from your destiny, and, with doing so, ending the final hope that could save Equestria in the future. My biggest regret is that I was afraid of my own reaction to informing you of this, and did not actually tell you in person.
Regretfully Yours,
The Doctor.