Jack remembers being happy. He remembers when he was full of smiles and he had finally had everything he ever wanted. He was with Kate. All the complications on the island that kept them apart were gone. Sure, there were new complications. Like all the lies they had told but that didn't seem that big of deal after saying them for so long. But still, he was with Kate. Sometimes he thought that he was dreaming, and that one day he was going to wake up on the island with her nowhere near him. But he wasn't dreaming.
Jack even had a son. Aaron wasn't really his son, but Jack was the closest thing to a father to Aaron. Not just the fact that Jack lived in Aaron's house for a couple weeks or that he helped Aaron tie his shoes or that he pushed him on the swing, but because Jack was his uncle. Jack could never get over this fact. Every time he looked at Aaron, all he could see was Claire. How could've he known that the blonde Australian was his half-sister? But Jack never told Aaron this. Nor did he hint. He kept his mouth shut and pretended that he wasn't his uncle and that Kate was his mom. He did for weeks but then he couldn't really do it anymore. Jack chooses not to remember that night. All he wants to remember is the happy times. But the thing about remembering is that it's just remembering.