I've encountered this so many times myself, and I still get burned on it, but mostly because sometimes the work you're doing is so technically interesting that

I've encountered this so many times myself, and I still get burned on it, but mostly because sometimes the work you're doing is so technically interesting that the "reward" is the work itself. Then, when you finish, you somehow expect everyone to celebrate and take it easy but then they shove more work at you. You couple that with maybe one day your project gets cancelled, and not due to the quality of the work itself but for other business reasons, and all of a sudden you just don't care anymore. This is something that just happened to me recently and I find it incredibly hard to get motivated to work hard on anything. I just can't "logic" my way into doing more than the bare minimum.