William Shakespeare

The fool doth think he is wise, but the wise man knows himself to be a fool.
― William Shakespeare

This quote refers to the pride of a fool for only a fool would think himself wise enough, or consider himself a wise man, and also to the awareness and humility that a wise man has towards knowledge and life.

The more a wise man knows, the more a wise man knows how much he doesn't know.

So, the one who knows is aware that he knows too little, knowledge is too vast and life too difficult to understand, as a result he (the wise man) feels like a fool.