Welll it's a man who can travel time - he falls in love with a woman, but he cannot control the fact that he travels time, so his relationship with her is very intermittent and brief. He appears, disappears, and reappears in her life at many different stages, and they do not have a grasp over when he shows up and leaves. The woman devotes herself to him regardless, but it becomes very disheartening being that she has to go through a lot of her life without him because of this. She spends a lot of her life waiting for him. (it's a sci-fi love story of sorts)

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The reason people find it so hard to be happy is that they always see the past better than it was, the present worse than it is, and the future less resolved than it will be.