Pruning or trimming of trees is just like getting a haircut. When you visit a barber and ask for a haircut he will do the needful along with minor cleaning wherever he feels the need. And he will not be chopping off the fingers, ears, eyes or nose. It is seen that in the name of just pruning entire trees are cut down with a tiny stalk left to re grow. This is not pruning rather it is personal benefit where the wood is sold in the market.
Pruning means that the main stalk is left and only the smaller twigs around are chopped and thereafter these twigs can be used for composting which makes very good manure. The main branch or the main canopy of a tree should never be pruned. All that requires pruning is the surrounding areas. Leaving the main trunk apart everything else can be pruned. The main reason of pruning is that a tree should look like a tree and not a bush that is low on maintenance. If the forest is dense then there is no requirement of pruning but if it is a public place where people do visit then pruning is a must to give a better look.
Whenever pruning is done always remember the example of barber. You go to the salon for hair cut and he will never chop your organs. So never be ruthless and violent in pruning plants as it is not good for them. Chop off only the small twigs for pruning so that the main trunk is safe and later a canopy can be formed with a healthy tree.