We were surprised at his words.中为什么用at而不用by?

at表示对于...,by表示通过...（一般表示方式）.例如：We were surprised at his words.对于他的话,我们感到很惊讶.She was overjoyed at that breaking news.对于那个爆炸性消息,她真是喜出望外.还有：Judging by hi...不是，be surprised 是系表结构，surprised不是动词的被动式，而是形容词。