Why did David’s harp playing relieve King Saul's illness?

Question

David's harp playing brought relief to King Saul from his mental illness. (Source: I Samuel 16.)

Why did the harp playing provide relief?

Answer 1

The spirit of H' departed from Shaul and there began to terrify him an evil spirit, Ruach Raah ... 1 Samuel 16:14

"Evil spirit" specifically. So what is music exactly? It's the Berur, selection and refinement, of good Ruach (spirit, notes) from out of the bad notes, as Rabbi Nachman explained.

A Kosher musician is able to draw holy Music from its source which is from the two Holy Birds. Thus Chazal teach, Why does the Metzora bring an offering of birds? etc. But opposite to everything in holiness there's the Other Side, Sitra Achra and there's such a thing as bad music. This is all explained in Likutei Moharan, Torah #3.

So David, in whom there was a strong revelation of Mashiach and of course being able to draw holy music, was able to play and to heal Shaul.

There are many connections to this matter, such as Mitzva Gedolah Lihyot b'Simcha Tamid, It's a big Mitzva to be happy, it's a constant Mitzva (that takes devotion), finding good points in your fellow Yisrael (Likutei Moharan #282), the Repair and refinement from Adam's fail, the Aspect of Shabbat (when good can be taken from bad but not vice versa which is Uvda d'Chol profane labor) etc. but that's a start.