He makes her promise one thing: he wants to go first. He doesn't tell her why, and she can't ask. There is something in his eyes that keeps her question to herself. Something fierce and quiet in those amber eyes that she can't help but promise him whatever he wants. And though she can't control destiny, she just nods her head. He smiles, and she's happy that the look is gone.

He never tells her why he wants to die before her. He won't ever tell her because even just the thought of being without her is painful. So he wants to go first. Even if it's selfish, he doesn't think he can take the agony of living without her smile. So he's happy when she gives into his request. He's relieved.

Which is why when the time finally comes, she feels guilty, and he feels betrayed.