Very interesting because I have had the same thoughts before about myself. I've noticed that my default reaction to a personal question from someone I don't know reeeaally well is the famous ISTP one liner. I just naturally don't like to share. I have no idea why. I do not let people in.
Last sentence, same here. But - and here's the problem - i don't have much of a choice whether or not they come in. Many people are invasive, and they never ever stop. I actually get sick over this.