Judging by the monster-wife the older shopkeeper seemed to have at the end, the story was more of a "Selkie" story.
The Spider-mom would attack the village if not given a sacrifice, until the monk sealed her in the lake.
The Spider-girl would take control of people using her webs, and force them to walk into the lake to feed her mom.
It seems that Suzuri was originally going to be sacrificed, but the Spider-girl had A: Fallen for him, or B: wanted to repay his kindness and thus fed Mizuki to the Spider-mom and took her role as Suzuri's companion in Mizuki's form.
Lots of legends, both Japanese and European tell of women that were originally spirits, animals, or both, that can take on human form to be the wife of a human, such as Selkies or the Legend of the Swan. Usually the myths end with the spirit wanting to return home, either leaving the husband or in some versions taking him with them.
As aficionados of old books, legends and myths, the two book-sellers would be familiar with these legends, as hinted at with the older man's "Wife" seen in the last few shots.
Whether there was any romantic reasons for the Spider-girl staying with Suzuri doesn't matter; most likely, she was using him to eventually find more food for her trapped mother. It's mostly left up to the viewer what the Spider-Girl's intentions were and why she is staying with Suzuri in Mizuki's form.