Like due to a misunderstanding they spent so much time apart and unknowingly hurt each other, even though they felt the same way. And later on when they met again as adults they had a bad impression of the other, and started a physical relationship under the wrong pretenses that felt bittersweet. They eventually did sort their stuff out but the path to get there felt a bit sad to me, so much time wasted

Its a good manga and all but it made me feel pretty sad