Trees in a forest are in a different setting than the ones growing over our bedrooms.  We trim trees in the urban/suburban environment because they live in close proximity to people, and we want to keep them there. That means they have different stresses placed on them and so we trim to remove deadwood or damaged limbs that can become a hosting place for diseases and insects, thereby extending the life of the trees.  We trim to help prevent storm damage and enhance human safety.  We trim to help trees develop healthy structure.  We trim to clear away from buildings, pathways and drives.  We trim to clear a vista.  We trim to decongest other trees or plants so one or both will still live.  We trim to help enhance the beauty and long term health of our tree friends. (ref: Tree Trimming, Ornamental Trimming, Shrub Trimming, Storm Damage Prevention Abstracts)