"You either trust him or you don't...checking isn't going to change your trust level"
It could. If you're suspicious about his/her behavior, look and find something, then that validates your reason to have been suspicious in the first place. If you're suspicious about his/her behavior, current activity, look and don't find anything, then that helps to put your mind at ease that maybe you were being a bit paranoid or that it was simply a coincidence that his/her shady behavior was nothing at all.
I... I really don't get what's going on. I'm about to be 27 next month, so maybe I just need a few more years to reach this zen level of having a relationship mentality of not being concerned if my girlfriend is out messing around on me. People cheat. It's in our nature. I'm sorry, but if I find my girlfriend doing some shady acitivities (keeping her phone on her 24-7, texting some individual at 2-3 in the morning, stepping outside to talk on the phone, lying about her whereabouts), that's going to make me suspicious. That's going to make me want to look through her phone and see exactly what's going on. If I find something, then I was right. If I don't find something I was wrong and that just makes my trust in her even stronger. I know that a grown person is going to do whatever they want at the end of the day, but at the same time I shouldn't just sit there and take it. A boyfriend or girlfriend at least owes the person who they claim to love the truth, a wife or husband isn't the only person who should be held accountable for their actions. So if my girlfriend was doing all of those shady, suspicious things (and my ex was), I was just supposed to ignore them because I'm supposed to trust her Even though she really was messing around? Interesting...