So you meet someone, you get married/make some sort of life-commitment, and the box goes unnoticed by your spouse for a few years (throughout the dating, the engagement, when they move into your place, etc). Then, during a move, your SO finds the box and asks what in it. You reply in an offhand manner. Your spouse then FREAKS THE FUCK OUT about why you've kept a box of memories from people you're not in love with anymore. They conclude that the only reasonable explanation for this is THAT YOU'RE STILL IN LOVE WITH ALL THOSE PEOPLE. A huge fight ensues.
Your SO is insane, right? There's nothing wrong with keeping some old letters, is there? WHAT DO YOU THINK?