The terms of discipleship are demanding. Financial questions loom large. My head assures me that He will, but my heart is not quite so certain. Physically, this is so. As someone has observed, people get ulcers not so much from what they eat as from what eats them.
Anxiety also steals emotional peace and removes spiritual assurance.
Telling us not to worry isn't very helpful. People who tell us that usually seem either unrealistic, uninformed, or patronizing. The Lord forces us to think about why we are not to worry. It is falling into the folly of the rich fool who believed that his life consisted of his possessions. But life is more than food and clothes, and God has promised us that He will care for us, much more so than He does for His creatures, the birds.