And he said unto them, Why are ye troubled? and why do thoughts arise in your hearts?
Note 2 at Lu 24:38: The disciples were troubled because they didn't believe. That's the reason we get troubled. We cannot control every negative circumstance that comes our way, but we can control ourselves. Believing God's Word will keep us in perfect peace regardless of what happens around us (Isa 26:3, see notes 2-3 at Joh 14:1).