Speaking of gravity, that's the explanation given to the logic of where and when he appears in time. He is pulled by a "gravitational force" to events that are emotionally significant for him. For example, he appears over and over again at the death of his mother by car crash. *Spoiler warning for the rest of this paragraph. Skip to the next paragraph to avoid. As described, he appears like an apparition to plant the proverbial seed of love in Clare Abshire as a little girl, and even after his death he appears to his own daughter on a school outing to offer comfort and wisdom. There's a lot of potential here to probe the emotional depths. The relationship that forms between the two time traveling members of the family — without the knowledge of the time-static Clare, and her subsequent feelings of being left out; or the infidelity of Claire as she cheats on Henry with a younger version of himself because the married Henry has gotten a vasectomy, an interesting statement on the inevitable decline of a man's virility. It's all fertile soil for drama, but nothing gets fleshed out. The scenes just keep coming without any reflection. As for the metaphysical can of worms that arises from the notion of multiple Eric Banas running around, these sorts of issues are sidestepped. Which is okay because you don't really miss them.
So are these two lovers meant to? Is there pairing an inevitability of fate? And is the end of love at the hands of death equally inevitable? Read the book and find out.