[SPOILER]
If you rewatched it or understood the parallel world concept the first time, then you will realise this story is way more enjoyable. It resonates with the audience because we all always wanted to change the past, right? Have you ever regret on a decision and you always wanted to take it back?
This movie illustrates what would happen when this kind of power is placed in the hands of a high school boy who just wants to have a happy relationship with a high school girl. And also brings out why "changing" the past is a bad idea.
Nevertheless it's not a perfect movie, nor a bad movie.
One example is the motif of trial and error. This movie's use of time travelling allows Norimichi to correct his mistakes to arrive to his desired ending. But even if he successfully corrects all of his mistakes he still cannot reach his goal which is to be with Nazuna forever. This makes both of them understand that they only can accomplish their end goal in their own world.
One of the themes in this story is acceptance. Norimichi has the power to travel to a parallel world to relive the same moment again, and he uses this to alter the course of the world, because he cannot accept the truth, for example he cannot accept that Nazuna will have to be taken away by her parents, so he goes to a world where he can dodge a punch from a grownup to try to prevent her parents from doing so, yet this world he travels to is still not the real world, it's just another fake world. In the end, he accepts that he cannot continue to live in this fake world and has to create his own path in the real world.
Symbolism is very important to the story. The story uses fireworks as a symbol of reality. Fireworks, in reality, explode in a sphere. But when Norimichi uses the crystal ball to travel to a parallel world, the fireworks do not explode in a sphere, maybe they are flat or the shapes are different. This symbolises the world they are currently in.
Too bad all of this is thrown down the drain because of the romance and intentions that wasn't explained much, this is why the themes, motifs and symbolism don't work as well as intended, because we don't get to understand the reasons behind Nazuna and Norimichi's affections. We also don't understand why Nazuna wants to run away with Norimichi knowing that they are still kids. Did she have terrible parents? That doesn't seem to be the case. Does she love Norimichi so much she cannot tolerate long distancing? That wasn't explained. If we knew what she was thinking in her head this story would have been ten times better.
Basically if you think a bit more while watching this and if you have a good attention span you might like this movie more. Don't expect it to be a compelling love story, enjoy it for its themes and ideas, also for its animation.