superposition principle

The principle of superposition is not "obvious" in any sense. It is an experimentally verified fact to certain accuracy. If we look at Newton's law of universal gravitation or Coulomb's law, we find that it does not in any way indicate that the net force is the sum of individual forces as if all other particles were absent. There is no reason for the net force not to be related non-linearly aside of being confirmed through experimentation. In simple words, indeed, we found that "it just works".