The fact is, it is always your choice, and you decide which tools you want to use on your road to career development. You don't want to take up someone's time. Maybe at work, as a preceptor. Or at home, with your children. Or maybe with a little sister or brother, or friend.
People mentor others for lots of reasons.
Perhaps someone helped them at a time when they felt lost and stuck, and, with the help of the mentor, they were finally able to achieve their dream. Maybe you remind them of someone they once knew and weren't able to help at the time. Or their intuition tells them they want to help you. Or your project closely parallels their own dream and allows them to vicariously experience the joy of completion through you. Maybe they admire you, your goals, and your vision.