Love, Disappearance and Regret

Chiaki's farewell

I sensed the same pattern when watching both Toki o Kakeru Shoujo (The Girl Who Leapt Through Time) and Fate/stay night. The main characters would meet and fall in love, but in the end one of them would disappear.

Moments before Saber's disappearance

As for the regret part: it's not like the characters in the anime truly regret it (they would move forward holding some sweet memories); I would be the one regretting that they couldn't be together (for some reason I always long for a 'happily ever after' ending).