There are many factors that come into play where tree removal becomes necessary, from diseases, to aging, to safety hazards. 

TRUNK DAMAGE:Vertical cracks, seams, dead branch stubs and large,older wounds suggest internal decay. Severe damage to the main trunk often warrants removal of the tree.

LEANING TREES:  Leaning trees are more of a hazard than those growing vertically. A Sudden lean indicates breakage or weakening of roots and tree should probably be removed immediately.

THE HEALTH OF THE TREE: If 50% of the tree is damaged, it probably should be removed.

A tree that is in decline can continue to survive for many years but will always have limited or abnormal growth and appearance