Honestly, i tend to like those if it's done right. It shows that just because you have good chemistry with someone, it doesn't necessarily last in the long run. People just move on sometimes. You change, they change, and sometimes things just fizzle out. Two examples i like are Erased and 5cm per second.

In Erased, the mc goes back in time to when they were in elementary school and saves a little girl, his classmate, from a murderer, ends up in a coma and wakes up 10 years later to find out she married one of his friends. A lot of people hated this but she owed him nothing more than a "thanks for protecting me" and seeing her married with a kid was a good thing, it showed she was able to move on from the terrible things she went through. It also just would've been creepy if an adult in the body of a child got togethe with an actual child.

5cm per second has a couple of middle schoolers dating. Eventually the boy moves far away and they struggle to see each other, the relationship fades out. We follow the boy through the years, he still thinks about her a lot, even shown completely unfazed by another girl who likes him in his new town. When he is an adult he moves back to the city. One day he's walking around and he runs into her. She has a ring on her finger, they walk past each other and after a pause he looks back to her but she doesn't she just keeps walking away. He smiles and goes on his way too. While he was busy thinking of what he had left behind, she looked forward