Ecclesiastes 3:4, 11

A time to weep and a time to laugh; A time to mourn and a time to dance.

He has made everything beautiful and appropriate in its time. He has also planted eternity [a sense of divine purpose] in the human heart [a mysterious longing which nothing under the sun can satisfy, except God]—yet man cannot find out (comprehend, grasp) what God has done (His overall plan) from the beginning to the end.