Why is my boyfriend so paranoid>?

its driving me mad. never ever in a million years would i do anything to hurt, disrespect or jeopardise our realtionship. i love him to bits. he looks in my phone every week searching desperately for something that isnt there. why cant he just trust me? i am nothing but honest and open with him but he's so jealous and paranoid. i found a text he sent to a mutual female friend a few weeks ago asking her "what she was wearing - be naughty - just between you and me" he was flippant about it and says it was just messing and as we both know here its nothing. is he like this because he is up to something?