He Loved Her Too Much:
He had only spent three years of his life with her. Three full years of adventure, excitement, tragedy, friendship, horror, anger and everything else life could possibly throw at them. They were inseparable throughout those three years and it was an eternity for her but for him it was mere fleeting seconds. He had never been so happy in his life and he hadn't truly comprehended the fundamental reasoning's behind it until it was gone. Until she was gone.
He loved her too much and after she was taken from his life forever he had no more love left to give.
After the adventure had finally drown to a close, he wanted nothing more than to go back to the beginning. The many years that had passed on torturously and agonisingly slowly was his own personal hell. He had wanted to join so much sooner but he couldn't. His job of protecting the city and his loyalty to his friends simply wouldn't allow him to. There wasn't a day that she didn't enter his thoughts for he had loved her too much to focus on anything else.
His friends eventually give up on him and move on with their lives, settling down and raising children of their own. Many girls are given her name as a sign of respect and devotion to her memory. But all he can think about is that she should be standing beside them with children of her own, the mother of his daughter. He simply loved her too much to say anything though for she would want her friends and him to be happy. Well he was halfway there and would always be so.
When the children of his friends are all grown up they eventually build up the courage to ask him why he never married and had children of his own. For once he doesn't snap at them or shy away from the question because even though it hurts like someone ripped out his heart. The cold stone truth is merely the fact of life. He answers because he loved her too much.
Naturally they frown but they are satisfied for they are well versed in getting the cold shoulder from him when it comes to a certain lost woman who died for them all. But he doesn't feel anything as they walk away except for maybe the black hole at the core of his being where she used to live. He loved her far too much.
It isn't until his deathbed that the subject of her comes up again. Annoyingly it turns out that after everything they have gone through he is the last to pass on out of his friends. He has gone to all their funerals and they each feel a knife in the heart but he has survived most likely out of spite from God because there is no logical explanation.
The question is why he left her on that planet to die for certainty. He could have risked everything and saved her and knowing his luck he probably would have succeed. So why didn't he try to save her if he loved her so much?
The answer was simple: He loved her too much.
In that instant that is the whole universe and then only them does he see all the possibilities of them getting out alive. They would make it, they always do but there is something in her sharp green eyes that tells him that she doesn't want to. She would forever hate herself and the monster she has become. She is no longer fully human and her humanity is her everything. She was the heart of the city that burnt like the sun and they would bask in her light for she would guide them into a better future. But she had been tainted from within, against her very wishes and she feels betrayed. Of course she understands why they couldn't let her die but he sees the look she gives him at says that she will fix their kind mistake.
And so that is why he left her to die a heinous end surrounded by machine monsters. He respected her wished even if that meant that he would spend the rest of his life hollow and unfeeling. He loved her too much to ever go against her choices and so he bid her farewell and never looked back.
He simply loved her.