I think it's interesting how many posters talk about privacy (or breach of) in relation to snooping or having something to hide. If that is the only part of privacy/sharing, I can certainly understand the confusion around why some people need it more than others.
I think the need for privacy is culturally influenced, as evidenced by the way many posters spoke of how their childhoods or past experiences affect their behaviour now. I think it is also a matter of temperament, and for those who have little need, it is difficult to imagine life another way. I find this is also the case for introverts such as myself, who usually need significant amounts of rest after heavy social engagements: extroverts don't need it, and are often puzzled by the need.
I have a high need for privacy, and sometimes I have had difficulty expressing this with a partner who doen't. I'm happy enough for him to know all my passwords, check my phone messages or email for me to see who's trying to contact me, etc. But I learned not to tell him things like my bank account balance, because I didn't want him to pass that kind of information along to anyone else in passing conversation. I guess that IS something to hide, but not because I'm guilty.... it keeps me safe! But for him, that wasn't something that was private, and didn't see it as breaching my rights: he didn't even think about it until I noticed later, and wouldn't think anything of it if I shared HIS information...