What compels people to snoop?

Strange question but I want to know what is going on in the minds of people that snoop. Like those people that start going through your drawers when your not home. Or if you set your camera down at a party and someone just takes it and cycles through all your pictures without asking. Or you tell them they can use your computer to check your email and the next thing you know they have done a sweep of your whole hard drive looked at everything you have. Honestly what the heck. Have you ever heard of asking? I hate it when people invade my personal space without asking. I just want to know what is going on inside the heads of all those snoopers out there?