Passion is a scarce resource. It gets spent for work, creation, training. When you spend all of it, you burn out.
Passion seems to be self-regenerating. I think mainly through helping others. You can't really recreate it by yourself, because you have to put in work to find the thing that gives you passion.
Spending passion without thinking about the return is wildly immature thing to do. Maybe it makes sense to use a framework of looking at passion like an investment - for whatever you spend it on it has to improve your life or give you something at a future date that is better than what you could get today. It's believing in the future, and optimistic people will have more potential energy to spend.
I had some childhood friends who I would look at and think "the things you could accomplish if only you cared enough", not in any sort of superior way. Maybe they were just doing the internal calculus of how to spend their passion, and were just more conservative and risk-averse. Why spend all that energy caring about playing the tuba when in 10 years it may not reward me with anything.
So how do you become optimistic? Is it natural or genetic or learned, or is it just a response to the environment. I realized that I'm trying to rationalize apathy, but my intention is to figure out where is the damned eternal spring of passion.