Mark
Mark would be peacefully unboxing the cases containing items meant for storage in the bedroom while you'd excused yourself to go to the bathroom. When he stumbled upon a large envelope that seemed like it had quite a few papers in it, he naturally felt curious, and so opened it. What he saw took him completely off guard; in his hand were love letters from when the two of you had been in high school. He'd break into the brightest of smiles, thumbing through the numerous sheets containing profession after profession of love. It would rehash all the old memories that had been pushed to the corner of his mind, from a first date full of nervous laughter and sloppy, inexperienced kisses, to leaving you for the first time to go on tour. He'd get very sentimental, and would want to share it with you when you came back.
Jaebum
When Jaebum found the envelope, he'd honestly just toss it to the side, not thinking anything of it until he finished unpacking the rest of the items. He'd take great care to decorate the room just the way the two of you wanted, the envelope long forgotten until it wasn't. He'd check it to see if there was anything important in it (otherwise he'd toss it out), and would be surprised to see the love letters it was full of. He'd want to sit down and read every single one of them, relive the words if he could. He'd cringe at the old poetry he'd written you, his first attempts at song writing, and melt at the loving words you'd sent to him in return. When he was done, he'd want to simply sit there for a while, and reflect. He'd absolutely made the right decision, dedicating the rest of his life to you.
Jinyoung
Jinyoung did not consider himself to be a romantic. Sure he had moments where his thoughtful nature expanded to include a dinner by candle light, or revisiting the area of your first date for anniversaries, but he wouldn't call himself romantic. However, he'd be forced to reconsider the opinion when he came across the contents of the envelope. In it were the meticulously well-preserved love letters the two of you used to send each other. He'd read in wonder as his eyes took in words he'd write you while he was away on tour, and ones you'd write him back that were equally full of adoration. Every letter seemed to be full of feelings he still felt, and he vowed to restart the tradition, writing you tangible, physical reminders of his love.
Jackson
Jackson would actually feel incredibly excited to see these letters again. He hadn't read them in years, but he knew he'd saved his. What surprised him, however, was that you had kept the one's he'd written you. When he stumbled upon the envelope containing the letters, he'd be very shocked and emotional. It meant the absolute world to him that you'd kept them safely for all these years. His choppy Korean from when he hadn't been very familiar with the language covered the sheets, and he was overcome with a sense of gratitude that you'd loved him for all these years. You'd stuck with him, through tour separations, fights, stage-induced exhaustion and injuries, and a whole lot more. He held reminders of every stage of your relationship in his hands, and he was proud of each and every one of them.
Youngjae
Youngjae was never one to verbalize his affection a lot. He'd get flustered very easily at the idea of saying everything he felt to you, and so it was just easier to write his thoughts down, and send you the letters. For these reasons, that's exactly what he'd done, unknowingly creating a tradition between the two of you. So when he found an envelope years later labeled "Youngjae♡", he'd curiously open it, only to be faced with everything he still felt for you. Over the years he'd made peace with saying all this out loud, and so the letters had lessened and lessened as time passed, until they finally stopped. Still, you kept them, and he'd feel a sudden urge to cry. These letters would show what a long way the two of you had come, from when he was to shy to be able to vocalize the reasons he loved you, to openly dedicating performances to you in stadiums full of people.
Bambam
Bambam always felt a need to shower you with affection. Unfortunately, he wasn't always able to be there with you, pressing chaste kisses against your lips, or holding you in long, lingering hugs. His best bet in times like those was to pen you his need for you, and hope you felt the same. So sitting on the ground of the bedroom of your new house with there letters strewn around him across the floor, he felt very touched at how much his words must have meant to you for you to keep them. These letters were a bridge he walked on to reach memory lane, reminiscing about the years past. The both of you were still young, but he doubted that even ten, fifteen, twenty years down the line he'd forget the day he forced himself to be brave and ask you for a date, or the way your cheeks flushed a delicate pink when you smiled at him and said yes.
Yugyeom
Yugyeom preferred to speak through sweeping romantic gestures, ones that involved restaurant reservations, or weekend trips to places the both of you had wanted to see. He stayed away from words, however, since to him, they always failed. He didn't know a single word that would be able to encompass the depth of his feelings, though god knows he'd tried. Which is why, when he saw his tries - the love letters he'd written you over the years - in the envelope, he'd be very surprised. He couldn't remember physically writing these, but as he read the careful words, he could visualize exactly what the letters were referring to. Sneaking out to see the Han river at midnight, kissing you when you tasted like birthday cake, asking you to be his. He knew, in that moment, that if he could remember these things vividly even after all this time, then there was something special about you two indeed.