Thank you so much for playing and sharing your thoughts! The game is indeed an endless loop. Your first experience playing the game is meant to be as similar as the character's experience after taking the pill. As you repeat the loop, you will look at the phrases on the walls, the pictures (and respective dates) on the frame, and the notes around the room, and you will understand what the game is all about. In this loop, the character wakes up, tries to escape, and then realizes that he locked himself inside, so he wouldn't need to face the whole world alone. In the end, when he takes the pill and locks himself again, everything restarts. For him, it's better to forget than to know that everyone and everything but himself is gone. So yeah, he's the one leaving the letter on the doorstep.