人心难懂毕竟你是你我是我
[rén xīn nán dŏng bì jìng nĭ shì nĭ wŏ shì wŏ]
'People’s Hearts Are Hard to Understand; After All, You Are You and I Am I.' Expresses a realization about human nature - individuals have distinct personalities, emotions, and thoughts that make them unique, thus not easily understood.