Tldr; if she's hot and you see her everyday, you'll get oneitis.

Disclaimer (this isnt my real life, just an example if i was prone to getting oneitis)

Everyday I follow my morning ritual. Clean my room, use washroom and make breakfast. I use the same bowl for cereal everytime and one pan for eggs. I love the routine and without it, something is missing. That something is oneitis. Obviously I'm not getting a hard on for my routines (although I could lol), but I would for the girl next door. She's someone I'll see everyday or I could if I wanted to. She's simply there and being her hot self. If she moved then I'd fall for that gym chick i see everyday at 9pm. If she bails then that cashier i see weekly. Even moreso within social circles. Stop seeing her and you'll stop caring or vice versa.

Now, if you have oneitis for a girl, chances are there's a bunch of guys who feel the same as you (unless it's a whale). If one of them is the top 20% where does that leave you?