I think that this (common) reaction probably illustrates a kind of over-correction among experienced problem-solvers:  the knowledge that not <i>everything</i> is linear (e.g.:  if you add a bit to the <i>area</i> of a circle, by how much does the radius change?) leads to the assumption that <i>nothing</i> is linear (or, at least, nothing that would crop up as a math-circle problem).