放心你离开我只会笑不会闹
[fàng xīn nĭ lí kāi wŏ zhĭ huì xiào bù huì nào]
It literally means 'Don't worry, I'll only smile not fuss when you leave me'. There might be unrequited love or self-reassurance involved, where one promises oneself to keep dignified in case the worst (someone leaving) comes true, which also has a hint of bitterness.