Therefore do not worry about tomorrow, for tomorrow will worry about itself... Matthew 6:34
Most people do not live in the now. Today is missed. Many either live in regret of things yesterday or live in anxiety about things tomorrow thus today is missed and simply becomes another day regreted.
When we worry, we use our imagination to create a negative future in our mind. We predict a bad tomorrow. Often our worry takes part in making the "imagined" bad tomorrow a reality. We tend to move toward our dominant thought. Worry can be a kind of self-fulfilling prophecy.
It is much better to focus on living today in a healthy and productive way. Today is a good day. It has many good possibilities. Take advantage of the moments to love and know the people around you. If you must allow your thoughts to drift to tomorrow, then imagine it being a good one. I think I'm going to go tickle my son now.