It's not simple that he keeps it, but I mean by the amount that seems to indicate he has A LOT of it.
I can understand keep around a few, but the amount of it makes me think of him as being a bit of a creepy... or obssessed.
@Jaylcs
I don't think it's creepy at all. It's more akin to being childish/girlish, something like how you want to collect everything the person you like gives to you as mementos. It's intended to be cute since girls are mostly the ones doing it, some girls even go so far as to make a scrapbook out of them. It's just his way of treasuring their moments together.
It ain't creepy like collecting hair samples or covering his walls with her photos or anything along those lines. THAT'S what obsession looks like.