Explanation:

Shankara wanted to go out for serving the divine mission of the Lord by propagating the true spiritual knowledge to convert the atheists existing in that time into theists. He was the only son and she became very old. No relative helps her since she was boycotted by village. In such situation, it is the bound duty of the son to serve her, since Veda says that mother is God (Maatrudevobhava..). But Shankara took the interpretation of this Vedic statement to mean that God is the mother. In Pravrutti, the first interpretation is correct since the mother stands first among all the souls. But in Nivrutti the second interpretation becomes correct because one is having several mothers in several births. Such relationship is the bond between souls and it varies from birth to birth. But the bond with the God is eternal in all the births and is the most important. Shankara was convinced about the second interpretation with the help of deep analysis of knowledge. No body could change His determination since He was complete in the knowledge from all sides. If the knowledge is partial, one will not leave the mother especially in such situation. Arjuna was not having the complete knowledge and hence hesitated to fight with his grandfather even though the Lord Krishna decided the war. Thus, if the knowledge is partial, one will either take a wrong decision or will be misled in to wrong decision by others. If the knowledge is complete, none of these two possibilities can happen.