The mind is like a sheet of white paper in this, that the impressions it receives the oftenest, and retains the longest, are black ones.
If you look at it ecologically, deforestation is high on the list of things which bring devastation. You cut down trees to build homes, for fuel, and you end up with no trees left, and you have to move on. If you take the earth as a whole, eventually there's nowhere to move on to.
What lies behind you and what lies in front of you, pales in comparison to what lies inside of you.