"Which of you can add a single moment to his lifespan by worrying?" -- Matthew 6.27 (EHV)
I find that a biting, embarrassing question, because of course, it is true. Being defensive and frightened and coveting has never really resulted in any gains. Partly we do it because we don't know any better way and partly because it's a habit. In either case, [Jesus] suggests another way: Seek the kingdom and his righteousness.
-- Walter Bruggemann, A Way Other Than Our Own
Put simply, worry is a lack of faith in your god. At first blush, that sounds a bit harsh, but I think Jesus would agree.
If your god is mammon, you anxiously follow the market and worry about the value of your 401k. If your god is a particular relationship, you fret over how quickly that person responds to your text messages/emails and lose sleep over the possibility that the relationship might end. If your god is your physical appearance and how others respond to you, you continually check your social media to see how many likes your selfies have and worry about how age and fading youth will affect what others think of you.
But if your god is the true God--Father, Son, and Holy Spirit-- you need not worry. Martin Luther encourages us in his Large Catechism, "Search and examine your own heart thoroughly, and you will discover whether or not it clings to God alone. If you have the sort of heart that expects from him nothing but good, especially in distress and need, and renounces and forsakes all that is not God, then you have the one, true God. On the contrary, if your heart clings to something else and expects to receive from it more good and help than from God and does not run to God but flees from him when things go wrong, then you have another god, an idol."
As Jesus said, “Do not worry, saying, ‘What will we eat?’ or ‘What will we drink?’ or ‘What will we wear?’ For the unbelievers chase after all these things. Certainly your heavenly Father knows that you need all these things. But seek first the kingdom of God and his righteousness, and all these things will be given to you as well. So do not worry about tomorrow, for tomorrow will care for itself. Each day has enough trouble of its own.” -- Matthew 6.31-34 (EHV)