Why do certain people use their left hand for writing while many others use their right hands?
Right hand is used in majority for writing, lifting, etc. Some people use their left hands for doing their routine work. Why such a change happens?
The human brain is a wonderful control center! It dicatates and processes everything we have coming in through the senses, those being touch, taste, sight, hearing, and smell. The conscious motor control of voluntary muscles is another department the brain has jurisdiction over. Brain research indicates people who are right-handed have the left side of their brain controlling that action, while people who are left-handed have the right side of the brain controlling that action.
When people are learning to write, as children, there is already enough "hardwiring" in the brain that has taken place that the children will take up a pencil in either their left hand or their right. The billions of neuropathic passageways in the brain are established through rigorous practice until the behavior is learned, or "acquired". A new link in the brain has been established that directly correlates to that physical activity, that is, being left or right-handed.
People in their later years who experience injury that damages part of the brain are able to reestablish new pathways through the cerebral cortex and regain the lost ability. Some people simply train themselves to write with the other hand. And then there are those who are ambidextrous, equally gifted at writing with either hand.