When an idea exclusively occupies the mind, it is transformed into an actual physical or mental state.
You don't argue with a four-year old about why he shouldn't eat candy for dinner. You don't punch a mentally handicapped guy even if he punches you first. And you don't argue when a women tells you she's only making 80 cents to your dollar. It's the path of least resistance. You save your energy for more important battles.
When we borrow trouble, and look forward into the future and see what storms are coming, and distress ourselves before they come, as to how we shall avert them if they ever do come, we lose our proper trustfulness in God. When we torment ourselves with imaginary dangers, or trials, or reverses, we have already parted with that perfect love which casteth out fear.