Barnes explains that "thought" originally meant "anxiety" and warns against anxiety that becomes "supreme concern," distinguishing between proper thinking about future needs and harmful anxiety: "See the fowls of the air... They have no anxiety about the supply of their wants... yet how few die with hunger!"
Matthew Henry warns against "disquieting, distracting, distrustful cares about the things of this life" and recommends: "Thoughtfulness for our souls is the best cure of thoughtfulness for the world." Gill forbids not "labour to maintain" life but warns against "anxious, immoderate, perplexing, and distressing thoughts" that are "dishonourable to God" and "uncomfortable to men."
Benson clarifies that Jesus distinguishes between prudent planning and "anxious solicitude" that "prevents our receiving or... increasing in the love of God" (BibleHub Commentaries).
