The challenge of parnossa

A person is unable to regulate the flow of his own blood, so he does not give it a thought. But if he could control his own circulation, he would never have a moment’s peace. He would worry constantly about maintaining the proper balance and live in terror that he might mishandle it and place his life in jeopardy. In regards to parnossa, G-d has issued a challenge to man, giving him enough leeway to delude himself into imagining that his prosperity depends on his own efforts.

(Rabbi Simchah Zissel of Kelm)