John Calvin on Matthew 6:31

This has the same object with the former doctrine. Believers ought to rely on God’s fatherly care, to expect that he will bestow upon them whatever they feel to be necessary, and not to torment themselves by unnecessary anxiety.

He forbids them to be anxious, or, as Luke has it, to seek, that is, to seek in the manner of those who look around them in every direction, without looking at God, on whom alone their eye ought to be fixed; who are never at ease, but when they have before their eyes an abundance of provisions; and who, not admitting that the protection of the world belongs to God, fret and tease themselves with perpetual uneasiness.