How do you answer "What do you have to hide?"

Many times when talking to people about privacy online, I always get the question, why do you care what are trying to hide?
Im not trying to hide anything, no one is, but that doesn't mean I want anyone ro know everything I do all the time.

I remember finding a thesis by a PHD student on this topic many years ago, I can't find it again it was around 35-40 pages if I remember correctly.

care everything hide people privacy question talking