When we meet a person, it is customary to ask whether he is keeping good health. But when Rama meets Sabari, His questions to her are different. He asks her whether she is eating less than usual. There was a reason for Rama's question, said Adur Asuri Madhavachari in a discourse.
Sabari was engaged in tapas. Those engaged in tapas eat little. Their thoughts are so focussed on God that they do not even think about food. For those engaged in penance, thinking of God and seeing His devotees amount to eating food.
Rama Himself clarifies why He has asked Sabari this question: He says He would not ask everyone this question; He has so asked, because Sabari has been doing tapas.
The word, tapas, itself is a name for the Supreme Brahman. Taittriya Upanishad makes this clear. Those who are engaged in penance have no thoughts of anything but the Lord Himself. Among those who think of nothing but Lord Narayana, Narada was one of the foremost.
That is why he was sought by Valmiki as Acharya. Narada not only entertained no thoughts of anything except Lord Narayana, but he could communicate the value of tapas to those who sought his help.
The ability to communicate is also important in an Acharya. Narada was an authority on the Vedas. An Acharya has to be thorough in the scriptures. He cannot have any doubt. If he does, he cannot be a good Acharya. So in knowledge and the ability to communicate that knowledge and in penance, Narada was unsurpassed, so he became the spiritual instructor of Valmiki.
Acharyas must be thorough in the Vedas and all other scriptures. In case, the Acharya happens to be a renunciate, there are rules about what he can and cannot recite. A renunciate cannot use words of blessing found in the first part of the Vedas to bless anyone. These are about how one gets wealth, healthy children. He can only recite the second part of the Vedas. This half deals only with Atma.
It is not that the first part does not speak of the Lord. It does, but it does so indirectly. The second half speaks of Him directly.
http://www.hindu.com/2010/08/30/stories/2010083060630900.htm