There are many things in our lives that cause us stress. While some things are legitimate problems, others turn out to be nothing at all. The worrying that we do results in a lot of wasted time and emotional energy.
Stress comes from thinking about the worst possible outcomes when we don’t even know what the future will hold. That’s why Jesus tells us to not worry about tomorrow. It doesn’t benefit us and won’t change the future. And in the context of this passage, Jesus reminds us that God will take care of us.