Boyfriends phone?

I seen a message on his fone from a girl he spoke to when he was single so i looked at it was nothing really then i asked him who she was etc and then he puts a password on his fone? A asked him why he says he doesnt want people going through his phone aka me! But i watched him and then when he was asleep i looked through his phone for the last week or so and there nothing to hide so why would he put a password onit? :s