He knows why. The question that has been eating away at him ever since he'd heard about his future self—the question that always had him wondering what the future would bring—for him…for her…for their love…the question that had him choose the power to see into the future…
Why does he go back? Why does he go back to die? Why does he go back to save her only to watch her die in his arms?
He knows the answer now. It's because he doesn't want to live another moment without her.
His future is their past. There is nothing for him in the present. The thought of taking another breath when she lies in a cold unmarked grave is painful to him. He needs to see her again, feel her in his arms again, if only for a short while. He is ready. He knows that they will only have a short time to be together, but it's all he wants. It's enough for him. More than he deserves maybe.
He knows that he's done it before. He wonders how many times. Has he ever had any doubts? He doesn't have any now. It's his destiny. He will do it again and again times infinity—for an eternity—just to be with her, to give her just a little more time.
How long has he known? He's suspected. When she had cried and begged him not to wear the suit and mask ever again because she feared he'd leave her…in their future…with his death…he suspected. There was nothing that could make him leave her. Except one thing. The one thing that he didn't want to think about. The thing that gave him nightmares. Kept him awake at night wondering if it was just a dream, or was it his power trying to warn him?
He should have known better. He can't help the guilt. If only he had tried harder to gain control of his power. If only he had stepped out of the shower room first. Why hadn't his future self warned her? He'd asked her thousands of questions about his future self. Most she hadn't had the answers to. But she had been able to tell him what his future self had said about why he'd gone back. To save her. To die. And yet, he hadn't been able to save her. He had had to watch her die in his arms. Watch as the light faded from her eyes and his dreams for the future turned to dust.
He had wanted to take her back to Vegas, this time without Nathan in the very next room, and maybe propose to her. He had begun to think that maybe there was something to married life, not that either of them had had good examples, but if it meant spending the rest of his life with her, then it was something worth contemplating. And there were so many little things that he had wanted to do. He had been looking forward to her teaching him how to drive, and the inevitable argument that would have followed. Which would have promptly been followed by rough and tender make-up sex. And he would miss waking up with her, watching her put on moisturizer after a shower, and asking him what bow he thought she should wear in her hair, only to get exasperated when he told her that it didn't matter what she wore, she would always look beautiful to him.
Why doesn't his future self tell her not to go to the community center that day and avoid Rachel and her vengeance? But can he give her that burden? Give them both that burden of knowledge? If he tells her, then she will spend the rest of her life knowing when she will die, the way he does now. It is an awful feeling. Knowing he will do it for the greater good doesn't help his fear. He doesn't want to die. He wants to grow old with Alisha and maybe have some children and grandchildren. But knowing it is impossible makes him feel trapped. It is a lonely feeling. He knows that when he goes back he will be watching from the sidelines, living like a ghost, visible, but invisible, never to be seen for who he really is until almost the end. He had been invisible once, and he doesn't want to be that person again. He doesn't want her to feel that way about her life. To feel trapped, and scared, and alone. He doesn't want her to be afraid of what the future holds, or worse, fatalistic about it. She had hope for the future once, their future, hadn't wanted to believe in the Terminator time paradox bullshit, and had almost convinced him not to believe in it either. He didn't want to take that away from her.
And he can only imagine that there must be some reason why his future self doesn't tell her; he just has to trust what he knows about what has happened in the past and embrace his destiny. He knows exactly what will happen if he goes back…but he doesn't know what will happen if he doesn't. If he doesn't, Alisha could live, but what if she doesn't? And his friends, Nathan, Curtis, Kelly, and even Rudy, they could all die, too. He could risk it, but if he fails, he won't get a reset button. He could choose not to go back, or to try to change the past, but he won't. If his short life has any meaning at all, it is because of Alisha, and he cherishes every moment with her, wants to relive every kiss, touch, fight, conversation, meal, text, look, date, emotion, scent, breath, dance, et al., ad infinitum.
He will go back for her. He will die for her. Everything is in place. He has gotten the power to travel the dimensions of time from Seth. He has said his goodbyes. He has packed the photo of them together in Vegas. The future is close. Time is about to begin again.