Earlier quoted context omitted.
You are misinterpreting the post. Derek isn't claiming that your decisions about what to keep private shouldn't be respected, but rather that you - the person who wishes to keep things private - might benefit from thinking twice about what is it you are actually afraid of, and whether it is all rational. Claiming that her diary was secret because "she wanted to" is really beside the point. People constantly "want" ma…
People constantly "want" many things which are harmful to them. In this case, Derek and his friend appear to want something which is harmful to them (almost completely transparency in all their activities, to the extent of revealing a private diary on request). People are just pointing out why this is harmful, and why he should think twice before proclaiming that 'secrets are just facts'. You can't ever take back hav…
As to the content of the post itself - there are obvious edge cases where releasing private information is harmful. I don't think Derek is claiming that they don't exists, or that total transparency is the best way to go. Let each of us think for ourselves what kind of information we should keep private because of real, rational and justified reasons, and what is based on irrational fears of exposing something that nobody cares about in the first place. I think you will find that often the majority of what people consider worth hiding is of the second kind.
People don't keep private diaries because they believe that "a growing lack of privacy is a serious problem in our society", they keep private diaries because they feel psychologically inclined to do so.