What made Albert Einstein invent mc2?

Answer 1

The theory of General relativity. Albert Einstein was trying to develop a theory that would better explain why the universe behaves the way it does. At the time most scientists were still using Newton's theories which did not accurately predict the movement of planets around the sun. To explain his theory and allow it to make predictions about the universe that could be tested and proved, Einstein produced a set of equations.

The famous E=mc^2 was produced to explain the energy to mass ratio. This equation has been widely used and shows that the energy of an object is dependent upon it's mass.

So really to answer your question in the most literal way possible, Einstein "invented" E=mc^2 to give credibility to his theories and provide evidence.