* edit: I still really like this series but I'm confused on the time-travel aspect...I mean in the future, Kakeru is still dead (they flash forward and back) so does that mean they failed? Or do they live in an alternate timeline in the future? I'm not sure how the series is headed but I hope the author doesn't drag out the plot. The pace she's set right now (and the time frame!) seem perfect and would make this manga so so so much better. Short, bittersweet, and to the point.

The story starts off with a letter that leads into the protagonist's past, present and future. From there, the story continues with two different time periods in which dictate the life of the protagonist as she interacts with her friends. Of her friends, most importantly her life-long crush is whom she deeply has feelings and regrets for based on her actions from her life ten years ago, which is being re-lived again by the protagonist (you'll know what I mean when you read the first chapter).
Obviously it's a shoujo-based manga, but mainly this story has something deeper within its plot and characters, something that I can't properly explain (and I apologize for that), but within that lies the message that truly struck me with such emotion (to the point that I honestly cried in every single chapter as the story and characters continued to progress), and part of @UsoSora's comment is what I'm trying to elaborate on, but with my own feelings attached into it.

It's not only the fact that while being given the chance to change your past for the hopes of erasing your regrets from your previous life that makes this story interesting and intriguing. The fact that these characters, who are a strong, loyal group of friends, are willing to put themselves out there to be truly helpful and caring for one another adds up to the true beauty of what is hard to find in our society today, and that is true friendship (in my opinion). And I can honestly say that for each of Naho's friends to sacrifice their time and feelings for the sake of Naho and Kakeru's relationship really emphasizes the emotional and precious relationship between them two and their beloved friends. And it just makes me so happy to know they're together because for Naho to eventually step out of her shell and truly express what she regretted not having done so really shows how committed and regretted she was, and along with such amazing friends, what more could she not be capable of doing? It is however painful at the same time because in between the story it shows the future Naho and her deepest regrets of what she did ten years ago, and it's so unfortunate that with such strong feelings she couldn't express herself honestly during that time. She totally deserves to have Kakeru marrying her, but it's also a sweet & sour twist to the story because in the future Naho, she ended up marrying Suwa, the guy who loved her for so long and sacrificed even his feelings just for the sake of their happiness. It's a lot to explain, but you'll know what I mean as you go on reading it. And with the way how I say "future Naho" and "younger Naho", I'm trying to identify them apart because future Naho is the 26-year-old while younger Naho is the currently younger Naho we see trying to follow up with the letter and trying so desperately change Kakeru's life with every chance and opportunity she's got.

Oh gawsh, the younger Naho that's in junior high right now.... I sure hope she ends up successfully changing those regrets and ends up with Kakeru. Even if her future Naho couldn't end up with Kakeru, as long as younger Naho can at least grant her future Naho's wishes in her place, she can truly deepen her bonds with both her crush, friends, and most of all, herself through strong commitment and sacrifice, and that's what deepens the story even more.
"Just because you can't change time and its past, present and future doesn't mean you can't change the person and people who lived during that time." ---> That, I think is the message that somewhat sums up my point, though I know my use of words kind of screwed over my original intentions, but whatever. Anyways, from there, [current] younger Naho may not be able to change her future Naho's past, but if she works hard to change her own present, she may end up creating a happy future, one that the both future Naho and younger Naho would have truly wished for.

the story skips from present to past. I was confused about this aspect but the mangaka introduced the idea of a parallel universe, which I found was more appropriate.The only reason that this story doesn't get a 10 from me is because the main guy seems so distant, and I couldn't really connect with him, nor understand his thoughts :/