Musar for Sanhedrin 197:19

Therefore, if one wishes to cleanse his soul, let him reflect upon the greatness of the issur of levity in several respects. For aside from the fact that he [the mocker] sins himself, it [levity] being one of the four things because of which one does not receive the Shechinah, as Chazal have said (Sotah 42a), he also causes many to sin, bringing them to multiply mockery. And the punishment of causing many to sin is well known, it not being granted one to repent because of this, as Chazal have stated (Avoth 5:18). And, aside from all this, involvement in idle talk, even if it contained nothing forbidden, causes one to lose the time that he could have spent in Torah study and the acquisition of the world to come. And he gives the impression that he scorns the words of the L-rd and the reward of the world to come, as stated in Sanhedrin 99a on the verse (Bamidbar 15:31): "For he has scorned the word of the L-rd" — R. Nehorai said: ['This refers to] one who could have studied Torah but did not do so.'"