One of my sibs went through a phase like this, too, where the answer to any non-factual question was "I don't know". In retrospect, I think it was the awareness that it is impossible to have all of the information about any one situation, such that one can definitively make the "right" decision. I seem to recall that it stopped around the time college started...I wonder if moving on to an instructional setting that acknowledged uncertainty helped.
I do think that it is a good idea to emphasize that, most of the time, we are choosing from two good options.
And for myself, you should see me trying to pick something from a menu...
