According to a verse of the Vedic mantras, parasya saktir vividhaiva sruyate: the Supreme Personality of Godhead has different varieties of energies. Sakti is feminine, and the Lord is purusa, masculine. It is the duty of the female to serve under the supreme purusa. As stated in Bhagavad-gita, all living entities are marginal energies of the Supreme Lord. Therefore it is the duty of all living entities to serve this Supreme Person. Durga is the representation in the material world of both the marginal and external energies, and Lord Siva is the representation of the Supreme Person. The connection of Lord Siva and Ambika, or Durga, is eternal. Sati could not accept any husband but Lord Siva. How Lord Siva remarried Durga as Himavati, the daughter of the Himalayas, and how Karttikeya was born, is a great story in itself.