Every excess causes a defect, every defect an excess.

Every excess causes a defect
这个懂吧？
every defect an excess.
这句其实跟上句一样，由于动词、含义都差不多，因此把谓语动词省略了
全称every defect causes an excess
这句也一样咯，Every sweet has its sour, every hardship its comfort.
后面那个少了个has

Every excess causes a defect, (and) every defect (causes) an excess. Every sweet has its sour, (and) every hardship (has) its comfort.
括号中的词省略掉了，以减少重复累赘。如果前后几个句子有相似的结构，那么后面的句子都可以采取这种省略的方式使句子显得简洁。

例如：
John is a lawyer，his wife (is) a cleaner.
Some of us study Japanese，others (study) English.
括号中的谓语可以省略。