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I completely understand how you feel, I'm also in a long distance relationship. Maybe the reason why is that he doesn't want you having his passwords is because it'll feed your ROCD? Since OCD feasts on compulsions (him reassuring you) maybe he realizes that if he give you his passwords it might cause more arguments, not that he's hiding anything but that he knows your OCD doesn't care if you can see all of his chats or not, you'll still need reassurance and possibly argue more. I can't really make assumptions since idk who he is but I think that's a possible reason.

I'm not sure if this will help or make things worse, but if I had someone ask for my password it would be an immediate red flag. It would imply a complete dimantlement of trust. Even if you don't trust your boyfriend, requesting a password might be a boundary crossed.