Well, see, there are a bunch of reasons. For one, because it is, in fact, a science fictional story and because it is, it is also a story about who we are as seen through another lens. Science fiction, or at least all good science fiction anyway, is an askew view of the normal, a way of looking into things by looking at what they are, are not, could be, could not be but are hoped to be, wanted to be, or even desperately hoped never to be, even as they seem more and more to be that way, at least a little further down the line. They are an extrapolation that points not forwards and not really backwards but at the now and what the now portends.
For another, because it is a time travel story, and to do a time travel story well, as this story most definitely was, one must take some serious forethought into it. Charles Yu has done his homework and shows us that he has without showing us that he did. I know, I know, it's all a bit roundabout, but it doesn't make it untrue. The best sorts of speculative stories adhere to Hemingway's Iceberg Theory. Yu's just happens to be one whopper of an iceberg. A bow breaker of an idea.
For a third, because in the bends and loops and twists that Charles pulls us through, he arrives at these truths that are so true that we know them to be so, know them to be so obvious that we nod along with them, both in acceptance and also in a little inward sort of anger, anger at ourselves for not having thought them before, or thought them so simply or so eloquently as he has. For instance:
"Life is, to some extent, an extended dialogue with your future self about how exactly you are going to let yourself down over the coming years."
See what I mean?
But why, beyond all that, did I love this book? Because it is the kind of story I love the most, a story about fathers and sons. My own impetus toward writing has a great deal to do with, and a great stemming outward from, the conception of the father figure and the role that plays in the construction of the son. What motivates the father also tends to motivate his son, in such a way that we want so badly to be what we know to be good in our lives: what our fathers are, or to be what we know to not be good in our lives: what our fathers were. This struggle between two sorts of ideals, two conceptions of who we are as blurred copies of what we came from, drives young men to greatness and ruin, and has for ages and ages. Stories of fathers are stories of triumph writ large upon the tableau of their sons, and the interaction between the two, especially here, most definitely here, echo the same forces that all young men come to deal with at some point in their lives, whether they prefer to or not. We are our father's sons, either in reflection or in opposition, but that voice drives us in ways we have trouble understanding or accepting. This story was a great example of such.
Pick it up. I highly recommend it.