Moussar sur Les Psaumes 103:1

And then he prospers in all his ways and in clarity of perception and the Holy One Blessed be He sends a spirit of Holiness within him and his heart rejoices and is filled with love of the Holy One, Blessed be He, and his soul is united with joy and reveals secrets and new intepretations from above, and all this because he reveres God, Blessed is He and is upright. And reason enters him. And this is what Solomon said : "My soul failed me when he spoke" (Eccl. 5:16). "Yea my body will rejoice when my lips speak right things" (Prov. 23:16). And thus said David : "Bless the Lord, O my soul" (Ps. 104:1). Because the soul which comes from above and strives upward knowing its own secret loves her Creator and eagerly assimilates His Commands, and when this soul reaches the veil (partition) and the degree which is suitable to it then she causes him (the owner) to rejoice with her hidden charms and brings him delight in her hidden chambers and at every moment his love longs for his soul and remembers it in the night as he lies upon his couch. Then God, Blessed is He, sends into her the longing for joy and the heart burns passionately from the great desire of love as it is said "I will rejoice in the Lord, my soul will be joyful in my God" (Is. 61:10). And happy is the soul that merits and experiences such joy.

But one must be especially careful about washing one’s hands before prayer, as it is written, “I will wash my hands in cleanliness, etc.” (Tehillim 26:6). And it is also written (Tehillim 103:1): “My soul shall bless Hashem and all my inwards His Holy Name” (that is, one must be sure that one’s intestines are clean as well).

Berachot 10 asks whom David had in mind when he repeated the words ברכי נפשי five times in Psalm 103. The answer given is that David addressed himself to the relationship between G–d and the soul. The Talmud lists five aspects in which the soul is like G–d. Just as G–d fills the universe, so the soul fills the body. Just as G–d sees without being seen, so the soul sees without being seen. Just as G–d sustains the whole universe, so the soul sustains the whole body. Just as G–d is pure, so the soul is pure. Just as G–d resides within an inner sanctum, so the soul resides within the most hidden area of the body. David said that the soul, because it possesses these five features, is suitable to bless the Lord who excels in these five areas. The soul judges the body because of the five aspects in which it is similar to G–d. The author cites proof how the five attributes of G–d possessed by the soul affect man.