To the fun theory, hedonic treadmill sequences.
http://gettingstronger.org/hormesis/
TL;DR stoicism with science.
Key idea: OPT, Opponent Process Theory: http://gettingstronger.org/2010/05/opponent-process-theory/
Research, PDF: http://gettingstronger.org/wp-content/uploads/2010/04/Solomon-Opponent-Process-1980.pdf
From the article:
"In hedonic reversal, a stimulus that initially causes a pleasant or unpleasant response does not just dissipate or fade away, as Irvine describes, but rather the initial feeling leads to an opposite secondary emotion or sensation. Remarkably, the secondary reaction is often deeper or longer lasting than the initial reaction. And what is more, when the stimulus is repeated many times, the initial response becomes weaker and the secondary response becomes stronger and lasts longer."
I went to school. That's a clear example of "repeated stressful challenges", and it did not produce any satisfaction or happiness.