why shouldn't you focus on the future?
I don't think it's necessarily that you should not focus on the future. I think there is a balance.
If you live your life focusing on the future, then you are not living your life right now. Same as if you were to live your life thinking about the past, you would be missing out on the things that are in your life right now.
Thinking about the future, it's possibilities, things you may have or may not have, worries and fears, can cause you to act in ways that you otherwise would not. It can cause you stress, anxiety, and often times, depression. On the other hand, not having any ambitions or goals for the future can also cause stress and anxiety, and again, depression.
I have known people who lived without having any goals or plans for the future, and while they were just living in the present moment, living each day as it came, they were also lacking a sense of purpose, they were lacking a destination, a goal. They soon feel into depression until they had to start taking better care of themselves.
There are people who make maybe too many plans for the future, who think too much about the future to the point where everything they do today is actually for tomorrow. As a result, they cannot enjoy anything, since every new day exists only to worry and care for the next day.
There is a balance, and for everyone that balance will be different.