We often find happiness in earthly, temporary things which cause that happiness to fade quickly.
God wants us to find our joy in Him because He is constant.
Praise the Lord! Praise the Lord, O my soul! While I live, I will praise the Lord; I will sing praises to my God while I have my being. Do not put your trust in princes, nor in a son of man, in whom there is no help. His spirit departs, he returns to the earth. In that very day his plans perish. Happy is he who has the God of Jacob for his help, whose hope is in the Lord of his God.
Psalm 146:1-5
Only God can fulfill our need for true happiness.