In an article on the site mindtools.com, I found this definition of burnout:"Burnout occurs when passionate, committed people become deeply disillusioned with a job or career from which they have previously derived much of their identity and meaning. It comes as the things that inspire passion and enthusiasm are stripped away, and tedious or unpleasant things crowd in."
Just reading that definition made me want to reach for the Prozac, but I think it captures the root of things. Speaking for myself, it is the crowding in of tedious tasks that are the greatest cause of burnout for me. I think deep down I have the constitution of an air traffic controller. I don't mind a lot of responsibility, a large workload, or urgent time crunches. I actually get revved up by them. But if you stick me with a bunch of tedious, rote tasks and I can feel my spirit dry up like a puddle of water in the desert.
Mindtools.com offers a burn-out self-test that is pretty interesting. It's an informal tool-not validated through controlled scientific tests-but it can be useful for exploring your own state of burnout. Check it out here.