只闻新人笑哪有空知旧人哭
[zhĭ wén xīn rén xiào nă yŏu kōng zhī jiù rén kū]
Translated to 'Hearing only the new ones laughing, there’s never time to hear the old ones weeping', it conveys the idea that when someone starts anew (for example, a new lover arrives), one forgets their past experiences and those involved in that.