The happiness of the son of Adam depends on his being content with what Allah has decreed for him.

 
[al-Tirmidhi, 2151; It was classed as saheeh by al-Haakim, 1/699, and al-Dhahabi agreed with him. It was classed as hasan by al-Haafiz ibn Hajar in Fath al-Baari, 11/184.]