What is the reason for the departure of the feeling of happiness and closeness to Allah after it had persisted for a period, and its transformation into depression and sorrow?

Happiness is attained through closeness to Allah and obedience to Him, as Ibn al-Qayyim described his teacher Ibn Taymiyyah as being among the people with the best lives, despite living in hardship. As for misery and worry, they come from being far from Allah, as He Almighty said: "And whoever turns away from My remembrance - indeed, he will have a difficult life." To remedy this, one must frequently remember Allah, make supplications, seek His pleasure, and turn to Him with sincerity. For happiness and a joyful heart are companions of the obedient. And Allah may test His servant to bring him closer to Him.