人心难留人心难懂
[rén xīn nán liú rén xīn nán dŏng]
'人心难留人心难懂' expresses two aspects related to human relationships: hearts (people) are hard to retain, and it's challenging to understand one's inner workings or feelings. This might come from someone having gone through experiences of failed friendships, romances, or other relational changes.