Indertia
Jean's life is centered around one family motto: For Mondstadt, as always. Being the Acting Grandmaster and managing the entire nation was never a part of her plan, but sacrificing everything you have for your country sometimes means sitting in the office all day. It's not the adventurous life of an elemental knight that she planned, but it's what her country is asking of her now. At the very least, it keeps her inside, far away from anywhere she could bump into Diluc after his return.
Diluc has no reason to do anything for Mondstadt ever again. After years away from his home, he doesn't have to be out every night, protecting the general public, defeating odd monsters, sniffing out conspiracy plans, and taking on the unfortunate moniker of the fire-slinging "Darknight Hero" (cringe), but he does it anyway. He and Jean aren't kids anymore, and they're certainly not friends anymore, but he's moved on from his past life. Nothing from that time is going to bother him again (or at least, that's what he's telling himself).
The story of how Jean and Diluc end up together, non-linear with events of their past.