The idea is really romantic and unique. It's cold; but it's important for you feel the 'cold' to romantically realise what 'warmth' is. That coldness is a vital part of the story (which's directly proven in a particular scene of the story).
Both the movies: 'To Every You I've Loved Before' and 'To the Solitary Me Who Loved You' follow the story of two studious students who seem to love each other but never fell in love. They married but there seems to be no love involved in that either. The girl had a nice attitude but it was mostly cold.
It appeared that the girl gave more importance to the child than her husband; this kills romance; some people don't see kids as a sign of romance (you know it's -> Japan).
There's a lot of science and nerdy stuff. Which makes its further unromantic and confusing altogether.
There's also a sister movie set in a parallel world which adds the romantic element to the whole story.
Some say that the idea is not realised properly, representation could be better; but it's really hard to come up with a better idea than spreading the story across two movies to make it easier to understand.
I know it takes great efforts and IQ to understand the entire mechanism. Pausing and rewinding movie several times is inevitable.
You've gotta trust me. There are movies with such nice story which fail at just marketing and don't get popular. It's one of them.
