So true tpe, falling in love with Ennis was the easy part. Getting Ennis to reciprocate that love equally proved to be the harder task. Though it took a while I think Jack evenvtually realized that Ennis did love him shown more through actions more than verbal. In SNIT it became crystal clear that Ennis felt deeply for Jack . More deep than he was ready to admit to himself.
You hit the nail on the head, aintfoolin. I do think Jack understood that Ennis loved him purely from his actions. He became a master of it, I'm sure, even though the beginning was quite tricky, as witnessed by the morning after.
How could jack have doubted after the reunion kiss? But that was 4 years later, and before that, perhaps he was unsure of how best to reciprocate.