Focusing on the future too much can be the cause of a lot of stress and anxiety, especially if these thoughts cause you to worry or be fearful.
Focusing on the future isn't necessarily bad. Making goals, plans, and even wondering about it is all a part of our lives. It gets bad when they stop being simple thoughts and wonders, and turns into negative thoughts and emotions, such as fear and anxiety.
What the future holds we do not know. Worrying about it is something most people do to a certain degree, but allowing it to change our lives, or even control it, is what can lead us down a bad path full of stress, anxiety, and a lot of negativity.
This is why one of the main pillars of mindfulness is to focus on the present and not the past or the future.
The past is the past, there is nothing we can do about what already happened. The future is untold, and we do not know what it holds. The present is where we are, it's where we can make a difference and where we have control.
Focusing on the present allows us to not give in to the stress of what the future may hold, and to not be upset of what was in the past.