A challenging problem is the most powerful inspiration for me. I really loves the following quote.
The greatest pleasure in life is in doing what people say you can't do!
One lesson that I am being able to learn is that whenever you put 100% effort in any work, you will succeed. Not 95% or 99% but 100% effort, it's like a step on a stair case, you can't seem to succeed if you raise to the 99% height of the step, but the moment you put that 1% extra effort, you are on the upper step all of a sudden. Also, I thinks "we never learn in a linear fashion but in jerks/steps".
And that is why if any problem arises that is challenging for me, "I say to myself, 'see you can't solve it, you are simply not good enough' ". And this always acts as a trigger for me and creates a wave of motivation.
(Life-Philosophy, Umm! maybe but I love it!)