I like this statement from this article "You must always be a fearless dreamer, but you shouldn’t undermine the laws of reality and life when you set your goals".
You're right enthusiasm without knowledge is dangerous, it can kill you. But again life without enthusiasm is worse than death, because everything in life will taste ugly to you.
The knowledge that our enthusiasm may always not be @ 100% is a good antidote against loosing enthusiasm completely.

Otherwise thank you chbartist, keep up the good work.