Imam Munawi gives a detailed explanation of the spiritual death of one’s heart as he states, citing various Imams,
“Excessive laughter leads to a hardening of the heart, which in turn leads to heedlessness [of the Divine], and the death of the heart occurs not except by heedlessness.”
He further comments, “Getting accustomed to laughter distracts one from reflecting on matters of significance…one who laughs excessively does not have a respectful demeanor [hayba]; he does not command respect at all. One who is characterized by it has no meaningful thought nor worth.”
He adds, “Excessive laughter and excitement regarding worldly affairs is a lethal poison that flows in one’s veins and removes from the heart fear [of divine punishment], sadness [over one’s sins], and remembrance of death and the terrors of the Day of Arising; this, then, is the death of the heart. ‘And they rejoice over the life of this world, yet the life of this world with respect to the afterlife is nothing but temporary, paltry amusement’ (Qur’an 13:26).”
Elsewhere in his masterful commentary, Imam Munawi states, “Laughter that kills the heart is that which occurs due to rejoicing over this life and being prideful in one’s joy thereof. The heart has [spiritual] life and death—its life is by continuous obedience, while its death is by responding to the call of other than Allah, such as of one’s ego, stubborn whims, or Satan.”
Finally, Imam Munawi states that the reason why excessive laughter kills the heart is because the root of excessive laughter is love of this world, which is the cause of every sin; and once the heart is dead, it does not respond to Allah when He calls him [to obedience].
[Fayd al-Qadir Sharh Jami’ al-Saghir]