When should one decide to change his/her career?

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In our life, we have many priorities. One of the most important priority can be profession, our work. I have seen many people changing professions after doing well in one? E.g. A friend of mine after doing an M.B.B.S degree, worked as a medical surgeon, had a pilot course and now works as a pilot for a well known Airlines in India? What do you think of this? What might cause such a major change to take place?