Today's challenge was quite exciting. I am a very enthusiastic person by nature so you can imagine that I was able to relate well to the challenge.
I could not agree more with Emerson's statement that "nothing great was ever achieved without enthusiasm." I believe that it's quite hard to fake enthusiasm, so with that said, here's my response:
In order to be genuinely enthused about something you have to be intrinsically motivated to do it. Plain and simple. That's all it comes down to for me. If you are tempted by the dangling carrot that is an extrinsic motivator, then it becomes hard to justify working when that motivator is taken away.
You may argue that your job has no intrinsic motivators... So basically you're saying that it is very hard for you to be passionate about what you do. To that I say 2 things: 1. False. 2. There are people out there who have seemingly more mundane jobs than you do and are far more passionate.
Find something to motivate you about your job (other than money). Something that will make you want to stay late or work weekends for no extra pay. It doesn't have to be your entire job (ideally it would be) but maybe just a small part of it. Find it, if you look hard enough you will.
Let me know how it goes, good luck.