Ennis was convinced that his fears were real. When he told Jack about the story of Earl and Ray, he called himself trying to protect him. To prevent it from happening to them. But somewhere along the line some one failed Ennis first. His dad maybe? Those images of Earl dead, were seared into his head at 9? years old and he was told by his dad (or shown) that this is what happens to guys who dare live together or are gay. .a cruel deed to force this on a small child. It was a failure on his dad's part not to teach him tolerance , acceptance, and love , not shame and guilt.To fix it instead of standing it. He just was'nt as strong as Jack or mentally prepared for such a deep, loving relationship due to this. I think Jack "got it" and loved him even more for it.
He obviously felt he'd failed Jack in the end. IMHO