I don't believe that motivation or passion is finite. I do not believe either commodity is like a gallon of milk and that one morning you have none left for your Lucky Charms cereal (not that I have had that experience or anything).
I think we burn out, or perhaps a better way to describe it is to say, we need to recharge. We need to recharge, regularly. Far more than we actually do. But our capabilities for motivation and passion are far from finite. They are only deepened by our ability to push the limit. Growth comes from extending ourselves until we think we can no longer extend. To push our motivation and our passion beyond where we thought our breaking point was. Every day our bodies must rest and perhaps every so often our motivation and passion must also bed down for the night. We must rest in order to perform at our highest capability.
But here's the thing that gets to me - as a leader - how does one know when too much is truly too much? How do we push the upper limit of motivation and passion, then encourage rest without stepping on or stifling growth in others and ourselves? Where is the line? How do we prompt those around us to recognize the capability and progress further and stronger, but maintain themselves - to not lose every bit of who we are without saving just a bit for ourselves?