1)
What is "Toras Picha"?
1.
Malbim: It is the reasons for Chukim, which are known only from Your mouth, for "mi'Piv Da'as u'Sevunah" (Mishlei 2:6).
2)
Why did he say "Tov Li... me'Alphei Zahav va'Chasef"?
1.
Radak: Others afflict themselves and exert in order to acquire money. I afflicted myself and exerted my body in order to acquire Torah, for "it is better to me than thousands of gold and silver [coins]."
2.
Mishnas R. Aharon 4 p.18: David had great Simchah [when learning], for he knew that every word that he says with his mouth, also comes out of Hashem's mouth at that time.
