The question in the title is akin to asking where the equation of motion of a free falling object L(t) = a + bt + 1/2 gt^2 in 3-space come from? then discovering that the “objects fall with constant acceleration” rule. Similarly, we derive the Seiberg-Witten equations (which also have a linear part and a quadratic part) from the deformation equations of an “isotropic associative submanifold’’ of a complex G2 Manifold. For this, we will define the notion of complex G2 manifold, and notion of complexification of a G2 manifold (this is a joint work with Ustun Yildirim).