The linearity of the Schrdinger equation is a consequence of the more general superposition principle. This principle states that causes add up linearly towards effects and it is postulated.
it seems that linearity of the quantum-mechanical evolution (aka Schrdinger equation) is a vital part of the structure of the theory. Nevertheless, we can never justify the linearity completely, the main reason for it is "it just works". But that does not preclude the possibility of a paradigm shift including the introduction of non-linearity.
