On one hand, challenges make me think [almost] everything is possible.
With enough force of will and perseverance, there are very few things one can't accomplish.
On the other hand, every challenge I faced taught me I'd never feel truly content.
That twinkle of satisfaction is soon clouded by a bigger challenge or, better put, the real one.
Great, you passed this tough exam, in 2 weeks you have this other one, which is the real badass. Oh, hey you got a degree, now good luck getting a job. Awesome, you quit smoking, stopped eating meat, what about dairy products?
Is there even an end, a point?
I learnt to approach challenges a bit like the horizon. You advance one step, it gets one step away. It makes you advance, which is great. At the same time, it's not meant to be reached, so take it easy.