Being an open book could make us over exposed or vulnerable to unwanted influences.
That's one reason why you wouldn't want to be an open book all the time.
Another would be to protect people close to you. Say you had a bad day at work. You may want to shield your family from your job's frustrations, fights, insuccesses, if they don't need to know about them. So you put on a mask or more when you come home to hide your feelings and thoughts that may still be with you from work. In time, they may go away, being with your family, but without a mask they would notice right away that your day was awful.
So true! Not everyone close to us needs to experience even remotely that we had a bad day. Sometimes, it gradually disappears like a mist when we're surrounded by family :)