R. Yonah: The heart always rejoices. It feels and is aroused more than all limbs. When your lips speak straight matters, the Simchah will be so great that also my kidneys will rejoice.

Orchos Tzadikim (Sha'ar ha'Simchah DH v'Zehu): The Nefesh is from above. When it understands its Sod ¹ , it loves its Creator and obtains His Mitzvos.

Malbim: If you reach a higher level of Meisharim, that you know the laws of straightness in Binah, you will not need at all a war of the heart. Straightness will be a Kinyan in your heart. Yashar is one whose nature leans to straightness. He understands the laws of straightness with his Binah. He gets a clear knowledge to speak it with his lips to others and teach them. Lips represent Da'as. Then my kidneys will rejoice. This is a higher level than Simchas ha'Lev. (a) Alizah is dancing amidst great Simchah; the Simchah is revealed in action, just like the laws of straightness were revealed to his son to teach them to others. (b) The kidneys are closer to action than the heart - the heart thinks and the kidneys counsel. Counsel is after thought.

Rashi: Since Chacham Libecha, [your lips speak straight matters].

Refer to 23:16:1:1-2.