Bf too secretive?

First off I do not believe my bf is cheating, however I do feel he’s not normal in how secretive he is. He deletes message threads (except for message threads with me) all the time. Like almost every day. He says because if he doesn’t delete them it will slow down his phone (he has a new nice android phone and doesn’t have that many apps or things in his storage, so I just don’t believe that). I don’t bother him about it though like it’s whatever I guess.

But yesterday I saw he was messaging this guy friend he hadn’t spoke to in years from the state he use to live in, on Facebook messenger and he got mad when I looked over at the messages (and I never do that) So I stopped but I was just curious about this guy. I asked what they were talking about and he angrily said nothing. Then later I did snoop on his phone (that was wrong of me I know I know) and the messages between them were deleted. I searched the guys name and the messages were gone. But his messages with his family and me were still there. I just feel like what does he have to hide? Why does he do this? Should I be concerned?