What do we learn from "v'Samta Sakin b'Lo'echa"?
Rashi citing Chulin 6a: When you learn from your Rebbi, contemplate what is in front of you. Ask a question only if you think that your Rebbi can answer. If not, refrain from asking.
Rashi: If you see that the ruler is stingy, do not eat from him.
R. Yonah: When they bring in front of you delicacies and they are pleasing to you, do not fill your stomach with them, lest you destroy the stomach's power. [Overeating] is the head of illnesses. Also, one who is drawn after pleasures, he separates from the ways of the Nefesh of Chachmah. "Tzadik Ochel l'Sova Nafsho" (13:25). If you enjoy a meal, hold back from it (Gitin 70a).
Mesilas Yesharim (Perek 11): One should consider forbidden foods like poison, or food in which poison was mixed. It is literally poison for the heart and Nefesh. Who will be lenient in a place where there is concern for Isur?!
Malbim: Do not eat much.
What is the meaning of "Im Ba'al Nefesh Atah"?
Rashi: If you are gluttonous and desire to eat, it is good to stick a knife between your teeth. Malbim - this is to stop you from eating much.
Mesilas Yesharim (Perek 11): If you have Sechel.
