He does not Take Sins Lightly

From Sahl ibn Sa’d radhiyallah hu ‘anhu said that Allah’s Messenger, Muhammad pbuh said: Beware of sins which are treated as being minor, just like a people who encamp in the centre of a valley, so someone brings a stick of firewood and someone else brings a stick until they are therefore able to bake their bread. Likewise sins which are treated as being minor and for which the person is taken to account will destroy him.
(hadis narrated by Ahmad, at-Tabranee, as-Saghir and others)

So it is just as the noble companion ‘Abdullah ibn Mas’ood said: The believer sees his sins as if he were sitting beneath a mountain which he feared was about to fall upon him, whereas the wicked sees his sins like a man who finds a fly settle upon his nose, so he does this (one of the narrators said: He brushes it away from his nose with his hand)
(hadis narrated by al-Bukhari)

The Muslim should realize the Greatness of his Lord, the One free and far removed from all imperfections, and His tremendous Power and Might, and he should not think of the sin as being small or great ! Rather he should think of it with regard to the One he is disobeying !

Those sins which are ignored by the weak hearted cause their destruction by removing Iman and fear of Allah from their hearts !

When a person regards sins and acts of disobedience as something slight then he is one who is oppressing his own soul, and that should never occur, since The Muslim Does not Commit Oppression.

A Time for this and a Time for that

From Hanzalah al-Usayyidee who said: Abu Bakar met and asked: How are you O Hanzalah? I replied: Hanzalah is guilty of hyprocrisy! He said; free is Allah and far removed from all defects! What are you saying? I said: When you are with Allah’s Messenger pbuh and he reminds us of the Fire and Paradise it is as if we are seeing it with our own eyes. Then when we depart from Allah’s Messenger pbuh and attend to our wives, our children and our business, then much of this slips from our minds. Abu Bakar said: By Allah we also experience the same. So I went with Abu Bakar until we entered upon Allah’s Messenger pbuh, I said: Hanzalah is guilty of hypocrisy O Messenger of Allah pbuh! So Allah’s Messenger pbuh said: And how is that? I said: When we are with you, you remind us of the Fire and of Paradise and it is as if we are seeing it with our eyes. Then when we depart from you and attend to our wives, children and business then much of this slips from our minds. So Allah’s Messenger pbuh said: By Him in whose Hand is my soul if you remained continually as you are when you are with me and in remembering (Allah) then the angels would shake hands with you upon your beds and upon your roads. But O Hanzalah, (there is) a time for this and atime for that, (there is) a time for this and a time for that, (there is) a time for this and a time for that.
Hadis narrated by Muslim)

So both of this times are regulated by the order of Allah, as has preceded. His time for serious matters is regulated according to the Book (Quran) and the Sunnah, and his jesting is kept free of anything forbidden and accompanied only by that which Allah has prescribed.

So he does not, through negligence, allow his jesting to become a way in which he falls into sin. Indeed how he could he do this when He does not Take Sins Lightly.