Checking your mans phone!?

So I've been seeing on here a lot lately about ladies checking there mans phone. Checking history putting apps on the phones to have all his texts and stuff sent to you. I just could not even imagine doing this kind of stuff I have full access to my fiancés phone and he has full access to mine but we don't go through it.  Like what is a relationship without trust ? How are you guys really in a happy loving relationship if your that insecure and constantly checking your S.Os phone?