Why is my boyfriend secretive with his phone?

He quickly swipes off pages on his social media, when i walk up to him, last night i walked towards him and he quickly swiped across his phone, why would he do that, he says that i do it sometimes and he's right i do but thats only when im talking to my mum or sisters about him when weve had an argument but he doesn't talk to his mum on instagram, so who could it be that he doesn't want me to see, weve had multiple arguments about trust because of his behaviours and he gets really annoyed saying what more can he do, he says he's never done anything to me, why can't you trust me, he gets really upset and angry, what do i do, its just his behaviours that i can't stand, weve argued about it before about why he hides his phone and he says because he's having private conversations with his friends.
Why is my boyfriend secretive with his phone?
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