Love Letters World's Apart

After testing out a new spell that opens a portal to a new world, Twilight curiously sends a letter to it hoping for a reply. When alas she does, it begins a chain of letters exchanging information between her world and theirs. But with every passing letter, she finds herself sharing more and more of her emotions and the other does so as well. Just what is this creature that makes her heart grow fond of them letter after letter?