Why is that?
It is because the kind of questions you ask determines the kind of life you are going to live. Your questions trigger a set of mental answers, which may lead to actions or inactions, based on the choices you make from the answers you have obtained. Remember, your life is always the sum of all choices you make in the process.
The bottom line: knowing yourself well holds the key to knowing others, as well as understanding the reality of all things; this mindfulness of self is true human wisdom.