Most Helpful Girls
It’s completely fine if they feel like being single 😐
I think so, yeah. They're memories. And once we're older, maybe even a bit too old to remember, I guess it would be nice to have stuff like that so that you could remember your past. Just like I wouldn't expect him to throw away all the pictures he has taken of people who were once in his life, but aren't anymore. What's most important is how he treats the letters. If he keeps going back to them and obsessively reading them, then obviously I wouldn't be ok with it. But if he keeps them in the box stuffed away somewhere, simply because the memories were too precious to throw away, I would understand completely. Saving stuff like that doesn't automatically mean that you haven't gotten over it or that you're still secretly in love with the person who sent those letters. It has sentimental value even if things didn't work out.
Most Helpful Guys
The past relationships helped us (not by design) to become who we are today. You can't, and you shouldn't, forget your past. Those letters are mementos of the past, like your old high school yearbook. She doesn't need to hate every guy who was in her life before me and she doesn't need to forget them.
HOWEVER. . . if she is pulling those letters out every day and re-reading them. . . "Houston, we have a problem!"
Sometimes, looking back at a relationship you were once in can help give you insight of what to do and expect for your current relationship. Sometimes, you'll have good memories of them and want to look back at the moments you shared with them. There's nothing wrong with wanting to keep contents from people you loved once before; it's only a problem if it affects your current relationship. I trust my girlfriend enough to let her keep her drawings and letters of her ex as a memento of the times they shared together because he was a respectful and good man, even if his temper caused a lot of problems in their past relationship together. You shouldn't have to force your partner to get rid of something unless it negatives impacts either of your lives or the relationship in general.