When you are not attentive, not here and now, your present peace of mind may be disturbed by some oncoming future event for no good reason. For example: Remember when you were in high school and you had a very important test coming? It was Friday and the test was to take place on Monday. That night you went to a party and enjoyed the evening for several hours when it happened! What happened? Suddenly, thoughts about the impending test darted across your mind and your night was ruined. You tried not to think about the test but to no avail, your present happiness was destroyed by the thoughts of a future event.
Understand that if I were to ask you how you feel, I am not asking how you felt yesterday or how you believe you might feel tomorrow. I am asking you how you feel at this very moment. If you were to answer me by saying, "at the present moment I feel fine," then you are fine because now is all that exists. Tomorrow will always be beyond your grasp and yesterday is forever gone. All you have is now. It is the only reality.