He believed he loved her,and she loved him,it was as if fate

这个应该是虚拟语气的用法.由as if/though 引导的方式状语从句常用虚拟语气,其用法为：
发生在主句动作之前：从句采用 had + 过去分词
与主句动作同时发生：从句采用 过去时(be 用were )
发生在主句动作之后：从句采用 would / could / might / should+原形动词.
而命运把他们两个牵在一起是发生在他们两人相爱之前,所以应该用 had + 过去分词.