Rashi: When one rejoices in his portion, his face illuminates.

R. Yonah: Simchah of the heart cures one from his illness. Gehah is a cure, like "v'Lo Yigheh Mikem Mazor" (Hoshe'a 5:13). This verse is next to the verse of the father of a Kesil and Naval, to teach that a Naval's father is missing great good - Simchah, the chief cure of the limbs - "v'Lo Yismach Avi Naval."

Orchos Tzadikim (Sha'ar ha'Simchah DH ha'Simchah): Simchah comes to a person amidst great serenity in his heart, without a mishap. One who attains his desire, and nothing happened to upset him, he will always be happy. His face will illuminate, his radiance will shine, and his body will be healthy, and old age does not come quickly on him.

Malbim: I already explained "Lev Same'ach Yeitiv Panim uv'Atzvas Lev Ru'ach Neche'ah" (15:13), that sometimes the heart rejoices and the Ru'ach is sad, if the heart rejoices externally, in good of this world and its success; his inner Ru'ach is lame. It is better to have a sad heart, and his inner Ru'ach will not be lame. It says here that a happy heart benefits only Gehah, i.e. Gevah (the external body) - "Lev Same'ach Yeitiv Panim." "Ru'ach Neche'ah" dries the bones - the internal, primary part of man.

R. Yonah: Great evil befalls a Kesil's father. Sadness rules over him; not only will it thin his fat flesh - it even dries his bones! This is even if he is healthy - all the more so he will not be cured if he is ill. This warns a father to rebuke his son, lest he will get sadness from his son's deeds.

Malbim: Refer to 17:22:1:3.