WHY DOES HE KEE DOING IT

So my boyfriend and I have been together a year and a half now we've had are downs and it's all about him deleting fb messages, Snapchat history, and texts. I keep telling him even if it's harmless conversation just you catching up with old friends there is no need to delete it. But he keeps doing it. It makes me even more upset that he deletes because now it seems like he is trying to hide shit. What do I do? We have had this conversation multiple times. I just can't get it through his head that deleting it makes it worse verses just leaving it unless you have something to hide. He says he doesn't but I don't know anymore