This is one of those cases where the answer is just another question. Why was Sanders so passionate about this? What did it mean to him? His brief biography doesn't really say much:
Existential crisis ("what does it all mean?") is one of the most common mental illnesses of all time. Pascal and Newton had it, and plenty of regular people do too.
http://www.marcsandersfoundation.org/about/founder/
And what went unsaid went with him to his grave.
Well, except that his son probably has some idea.