∞ Cities need more trees
On top of that it just looks better. I'm certain humans have genetic biophilia, which is why we love being in nature or taking walks in the forest. Having trees around us, teeming with birds and other life just feels good. Speaking of birds, trees increase the biodiversity of insects and other small critters in urban environments. It also gives birds a safe-haven from the deadliest hunter of all: the humble house cat.
I very much agree that cities just look better with trees. That's why I fell in love with Menlo Park the place I have been living for 9 years. It's a much greener city than most of other places in Bay Area.