Musar su Salmi 63:2

You already know that what is most desired in the service of G-d, may His Name be blessed, is desire of the heart and longing of the soul. It is concerning this that King David praised his portion saying: "As a deer yearns longingly for the water brooks, so does my soul yearn longingly to You, O G-d; My soul thirsts for G-d..." (Tehilim 42:1-2), "My soul longs, and goes out for the courtyards of G-d" (Tehilim 84:2). "My soul thirsts for You; my flesh longs for You [in an arid and thirsty land (Rashi-desert), without water]"(Tehilim 63:2).

This looking into will cause him to not grow lazy or lax in His service. For since it is impossible for him to repay G-d, blessed be His Name, for His goodness, he will feel that at least he can thank His Name and fulfill His commandments.

Behold, this is the precious trait which the Pious men of old, the lofty holy ones had merited to attain. As king David said: "As a hart cries longingly for the water brooks, so does my soul cry longingly to You, O G-d; My soul thirsts for G-d, for the living G-d; when shall I come and appear before G-d?" (Tehilim 42:2-3), and "My soul yearns, yes, faints for the courts of the L-rd" (Tehilim 84:3), "My soul thirsts for You; my flesh longs for You..." (Tehilim 63:2). All this due to his powerful yearning to the blessed G-d.