How do some people become champion athletes despite having physical disabilities? What causes someone to become anorexic, willing even to starve themselves to death?

What makes people make such sacrifices to come
out of retirement as a doctor or nurse, come to another state, leave their
families and safety of their homes to help others?

How do some people become champion athletes despite having
physical disabilities?

What causes someone to become anorexic, willing even to starve
themselves to death?