我非柠檬为谁心酸
[wŏ fēi níng méng wéi shéi xīn suān]
It conveys the sentiment that one might be bitter or melancholic for reasons others wouldn't understand, much like a lemon tastes sour yet is beneficial. It suggests a person feeling emotionally pained, possibly due to love, life struggles, or loneliness, asking 'If I am not a lemon (if my life isn't supposed to feel this bitter), for whom am I going through this emotional hardship?'