What is "Nichsefah"?
Rashi: It desires.
What is "Kalsah"?
Rashi: It craves. This is like "va'Tchal David ha'Melech Latzeis El Avshalom" (Shmuel II, 13:39).
Radak: This is a greater desire than Nichsefah. It is as if it wore out amidst great desire. Malbim - my heart is sick from love and yearning.
Mesilas Yesharim (Perek 7): What is most accepted in Avodas Hashem is desire of the heart and yearning of the Neshamah. David glorifies himself about his good portion.
Why did he yearn for "Chatzros Hashem"?
Radak: All of Yisrael stood in the Chatzeros, and not in the Heichal.
Why does it say that his heart and flesh will sing?
Rashi: They will pray for this.
Radak #1: The mouth sings with intent of the heart.
Radak #2: This is a metaphor, like "Kol Atzmosai Tomarnah" (35:10).
Malbim: Not only my special Nefesh longs for this - also my heart and flesh long to return to the Beis ha'Mikdash and sing Shir Hashem over the Korbanos. The physical mouth and lips sing with feeling of the heart.
Why does it say "Kel Chai"?
Malbim: He is the source of his life.
