My boyfriend keeps letters from previous relationships

How come my boyfriend keep letters from their ex-girlfriends?

My boyfriend stated that the envelop from their ex was at storage space and which he read it once again. But later on I experienced a sense he didn’t away throw it and I also ended up being appropriate he kept it. This strikes me personally as strange I have ever given him because he has thrown away all of the letters and cards. He stated he's got no explanation to help keep them, and that he does not keep letters and cards from individuals.