I recently asked this question to my students (it was actually part of a lesson) and here is what one of them kindly replied:
The problem is not if people are pessimistic or optimistic, but that we need to consider the future of our planet from a realistic perspective.
The Earth's climate has changed a lot in the last 20 years, and if nothing is done, life will be jeopardized.
Furthermore, our lifestyle is terrible because we are motivated to consume more and more.
As a result, we are depleting all our natural resources. One can not ignore the efforts of those who try to recycle resources and waste, but it is in fact not sufficient to stop the changes.
To sum up, the future of the world as we know it depends on our motivation and work. If we have the drive to change our lifestyle, the bases of our economic logic, which will soon change. That way, we will broaden our world view and be able to live on the Earth for thousands of years more.