This has been harder to find than I imagined.
I'm looking for any sort of phrase or quote that talks about how, very, complex interpersonal relationships are.
The best I've found so far is something like:
人情似纸张张薄，世事如棋局局新
which is okay, but I was wondering if there was anything better.
A common phrase for 'complex' is 錯綜複雜
It can be used to describe:
Example: 人際關係錯綜複雜 (interpersonal relationships are complex)
Example: 兩人的關係錯綜複雜 (the relationship between the two men are complicated)
Example: 案情錯綜複雜 (the case is complicated)
Example: 江南河道錯綜複雜 (Jiangnan's River system is complicated)
Another common phrase for 'complex' is '千絲萬縷' (tangled/complicated)
Example: 兩人的關係千絲萬縷 (the relationship between the two men are complicatedly tangled)
One more phrase related to the question: '人心隔肚皮 '
It means everyone has their own agenda. No one really knows exactly what other people are thinking.
Chinese sayings like '满肚密圈', '心知肚明' suggested people keep their intentions and plans in their '肚' (belly) which is always inside a person and hidden from views.
人人关系扑朔迷离
《乐府诗集·横吹曲辞五·木兰诗》：“雄兔脚扑朔，雌兔眼迷离，双兔傍地走，安能辨我是雄雌。”
