beholder是什么意思?

beholder的中文含义详细解释：
n. 旁观者
beholder的列举例句：
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。