Everyone always knew Louis adored Harry. It was rather obvious to everyone. Except for Harry himself that is. So he writes Harry letters because he has no inkling as to if Harry could hear him but he still engraves them anyway. Inebriated on the thought that maybe Harry loves him back. Because maybe if he can't hear him saying those three words millions of times a day he can see him writing them. So when things go downhill, Louis can't help but write them even more with the hope Harry will somehow give him a sign that he loves him back.