Trees depend on their leaves for their manufacture of food and when you remove more than to of the crown, you cut off its food-making ability. The tree's crown is like a sunshade that protects the bark from the direct rays of the sun and when you remove its top, you expose it to scalding and death. Leaving behind large stubs at the end of the limbs leaves them exposed to disease and insects. Think of it like leaving a large wound open on your skin and not protecting it with a Band-Aid. The weak sprouts that develop from the large limbs are weaker and more prone to disease.
Trees are a valuable asset.
A topped tree is just plain ugly and the tree never regains its original grace and character. There are some valuable points to remember about topping. The first is don't ever top a tree . Be sure to plant trees where they can grow naturally and won't interfere with wires or buildings. If pruning is necessary, consult a certified arborist or pick up a brochure and read about pruning.