beholder

n. 旁观者

She seems very plain but he dotes on her, but beauty is in the eye of the beholder.

她看上去相貌平平，但他非常爱她，情人眼中出西施。

I don't understand how he can think she is good looking, but beauty is in the eye of the beholder.

我不明白他怎么会认为她漂亮，不过情人眼里出西施罢了。