What causes people to worry about the future?

We are likely worried about some looming hardship.

Worrying over something that is going to happen will make our lives stressful.

After examining an upcoming difficult or seemingly disastrous situation, if there is something that can be done to improve on it, do it. If not, and the event cannot or should not be avoided, then accept the fact and start thinking about something else. Worrying about it will not only be a total waste of time but it will increase anxiety, and things will seem even worse than they really are.

"The secret of health for both mind and body is not to mourn for the past, not to worry about the future, not to anticipate troubles, but to live the present moment wisely and earnestly." Buddha (BC)