The word "spirit" has its etymological root from the Latin "spiritus" which means "breath".
The man is an indivisible unity of body, mind and spirit, and the latter has as its physical basis the breathing.
Breathing is a "wind" or "breath of life" that God breathed into man's nostrils at Creation (Genesis 2:7 "And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being.").
Breathing is a divine "wind" that accompanies the man in his earthly life. In ancient times, the breath and what it carried (the sound, the voice, the word, the name) contained life. In Christianity it is said that God breathed into the clay to generate the man.
The attention to the breath is mostly a reunion with God, a reunion with our physical base that sustains our spiritual life.
In difficult times, the re-encounter with breathing is a kind of a reunion with God. And when the breath is shallow, careless and fast we can also say that, somehow, so also is our relationship with God. Perhaps because breathing is shallow, careless and fast when we are inattentive and fast surface with our neighbors.