I relate to this. I don't know what it is either, but over the course of my 25-year career it's become impossible to summon that "will" within me. My brain just constantly asks "and what are we doing this for again?" which is anathema to sustained effort. I wonder if it's simple age; maybe this is just the energy of youth that evaporated. Or maybe it's becoming jaded; seeing how people will encourage their employees to burn it hard, for a modest reward that (in my experience anyway) often does not even materialize.