柠檬再酸也酸不过人心
[níng méng zài suān yĕ suān bù guò rén xīn]
This net name means 'Even if lemon is sour, it cannot be more sour than human nature'. Lemons are commonly known as something sour. Here it suggests that in some ways, human hearts/nature may turn out to be even more disappointing and hurtful than lemons. There could have been negative experiences that led to such expression of distrust.