Do you want or expect your "sweetie" to snoop through (your) personals? Facebook, emails, underwear draw, that taunting box at the bottom of the closet? These are questions each of us should ask ourselves (before) we start playing private detective. We also should take into account what could happen if we get caught.
People tend to "snoop" when things are going well because they can't believe things are so (great). They also snoop when things aren't going the way they want because there has to be a "underlying reason". The tagline on the TV show "Cheaters" is: "Exercise your right to be informed".
I suspect there are some people who just love going through other people's things period. Uncovering secrets makes their day. There are others who simply never trust (anyone) completely. However the real insecurities and trust issues have to do with ourselves not trusting we can select the "right" people to emotionally invest in.