Because in much wisdom there is much grief, and increasing knowledge results in increasing pain.

For all his days his work is painful and sorrowful; even at night his mind does not rest. This too is vanity (worthless).

All his days are in the dark, and he has much sorrow, pain, disease, and trouble.

He removing stones; shall suffer pain by them; and he cleaving woods asunder shall be endangered by them.

So, remove grief and anger from your heart and put away pain from your body, because childhood and the prime of life are fleeting.