Tóngbìngxiānglián, tāmen hěn kuài jiù chéng le hǎo péngyou.
Sharing the same troubles, they quickly became good friends.
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history
This idiom originates from 'Wu Yue Chunqiu' (吴越春秋), an Eastern Han dynasty text. It originally described people with the same illness comforting each other. The phrase captures the universal human tendency to bond over shared misfortune.