Eleven hundred years is a long time, more than four hundred thousand days. Every pony only lives oh-so-many of those days. Everypony but Celestia, she's lived them all.

Most ponies don't remember every pony they've ever met and befriended, some get forgotten over time. Everypony but Celestia, she remembers them all.

A letter from an old friend can be a welcome thing in all those many days.

Sometimes a letter gets lost in the mail.