少年不知笑颜为谁绽少女不知泪水为谁流
[shăo nián bù zhī xiào yán wéi shéi zhàn shăo nǚ bù zhī lèi shuĭ wéi shéi liú]
The young don't know for whom their smiles bloom or for whom their tears fall. This reflects the naive innocence of youth, often associated with unreciprocated feelings or love that's hard to express.