What is the reason for the occasional disappearance of khushu (devotion/humility) during prayer, recitation of the Quran, and supplication, and its return at other times?

The heart experiences periods of languor due to sins or heedlessness. To avoid this, a believer should grieve over their condition and seek means that help them achieve humility (khushu') and soften the heart. These means include: understanding the virtue and reward of humility mentioned in the Quran, seeking refuge in Allah from a heart that does not humble itself, feeding the poor and caressing the head of an orphan to soften the heart, promptly repenting when committing sins, frequently seeking forgiveness (istighfar), supplicating (du'a), and often remembering death and visiting graves.