The fact that they loved each other was merely proof that the fault lay not in themselves, in their behavior or inconstancy of feeling, but rather in their incompatibility; he was strong and she was weak.

词汇：

proof：证明

lay 放置; 铺放;在于

inconstancy：反复无常;

incompatibility：不兼容; 不一致; 合不来; 不匹配;

句子分析：

句子主干为the fact proof ... that 为the fact的定语从句。

not in ....rather in ...

【译文】 他们彼此相爱，这个事实只能证明他们本身没有问题，他们的行为或情感的摇摆也没问题。问题在于他们彼此不相容：他很坚强，而她太脆弱。