It's not always as simple as that. There are times where it's better to keep certain information from certain people no matter how much you trust them or like them. It's just better that way.

Some of us have had significant others with jealousy issues. It's not that we don't trust them, it's just better if they don't worry about silly issues like whose Facebook profiles we've been perusing. Not because it's a secret or because there is anything to hide, just because it helps the SO control their counter-productive impulses if they aren't informed on the matter.

That same kind of avoidance can be applicable in all other sorts of relationships. It's not that the people aren't trusted, it's just that there's no reason to know, and it will only cause damage if they find out -- not because it's bad or trust-breaking, but because the person's reaction may be problematic for completely different reasons (compulsions, or potential taint of future circumstance).

this is the old "you don't have anything to hide" mantra dressed up in more personal terms. people aren't perfect and they have the right to distrust their closest friends if they want to.

Of course they do. That's why modern operating systems have security.

It just doesn't make sense to insist that both (a) I want to leave everything unlocked and open; and (b) I don't want anyone to be able to see what's there.

You've got every right to protect your privacy from prying eyes. But if you want to do so, do it.