No matter how much you may say you "hate" people, at the end of the day, we all long for that feeling of closeness and reciprocated empathy with others. If you really think about it, some of your happiest memories probably include friends and family. We crave connection. In fact, we yearn for it so much that we engage in some of the most baneful behavior just to feel it, even if just for a moment. We share parts of ourselves that perhaps we shouldn't because we want others to deem us trustworthy — as if that is the key to attracting and retaining more people and more meaningful relationships in our lives.
It makes sense. Our relationships with others operate as isolated transactions in which we trade our time, energy and emotions for that of another person. Don't people bond most during those deep 2 a.m. conversations where they share secrets they ordinarily keep to themselves? Being an "open book" is easily one of the worst things you can do, especially if your underlying motivation is to retain more friendships. People are generally attracted towards things that have an air of mystery, a challenge. There's a certain appeal in having to work to figure someone or something out. If you hand everything to someone on a silver platter, they won't have to put work into understanding you.
The more you divulge, the more you leave open to interpretation. You create more opportunities to be vulnerable, no matter how confident you perceive yourself to be. Why do you do it? For confirmation? Because you, yourself, don't know who you are? Maybe you do it because you're so unsure of yourself, you need other people to tell you who are and, from there, you can pick and choose who you want to be. You should know who you are and correct others who misinterpret you rather than the other way around. In branding yourself as an open book, you set this standard of oversharing that people will then expect from you. In the event you no longer wish to live transparently, your decision to be more secretive could stir up misconceptions of ill will towards those in your social circle when, in reality, you simply want the basic right to privacy as everyone else.
In relationships, having separate lives apart is just as important as sharing lives together. No matter how much you may want to share every aspect of your life with your significant other, the same can be said about this as I mentioned above in reference to general friendships. If you know everything about each other and are not constantly growing in your own ways and experiencing new things in your respective lives apart, it becomes boring. The mystery and excitement disappear. If you always tell each other everything right away, you'll inevitably run out of things to talk about even quicker.
No one should know everything about you. While it's fun to share pieces of yourself and connect with others, setting your own boundaries is absolutely necessary for your own mental health as well as the health of the relationships you choose to maintain. Once information leaves your mouth, you lose control over it. It's sad to think this way, but that's just the way the world works. Filtering what you share with others is the best way to continue to work on yourself without completely being under the scrutiny of others.
You're in a bookstore. You sift through the aisles, eyes wandering, in search of your next good read. Do you pick up the mainstream novel that everyone has been talking about and has, therefore, been spoiled for you, or do you pick up the critically acclaimed novel that you know nothing but a glimpse at an interesting plot?