We all have something to hide from others especially the people we know. Something that may change someone's thoughts and perspective about you. But everyone is entitled to their right of privacy.
I find it extremely offensive when someone starts making it their business to try finding out what it is you don't talk about to anyone, even the people we claim to trust the most. Maybe it is not fully trust when we choose not to tell but at the same time, answers shouldn't be forced out of you.
If a person chooses not tell all their business to the world, why would it bother someone else unless they have malicious intentions? Why would some go out their way trying to figure out something that is not of their concern?
I think we should all afford each other the courtesy of leaving people their privacy, everyone has the right to choose who to show their innermost being and who not to. Is it not a shame, seeing someone try very hard to figure out someone's life yet they are not in position to neither help nor do anything about it? If I'm staying out of your business, why don't you stay out of mine?