A criticism I've heard a lot before for Nerdy Prudes Must Die is that it doesn't make sense that Steph and Pete would be what the other cherished most because they had not known each-other that long and I don't really think that the criticism is wrong, but I would like to offer a counter-point.
I don't think it's unrealistic I think it's just deeply sad. Isn't it sad that in these characters 18 years of life they haven't found anyone they connect with on the level they connect with each other? Isn't it sad that they never met someone who saw them for who they truly are? Steph before that had never met anyone who didn't see her as stupid. Pete hadn't met anyone before who saw him as genuinely cool. That's depressing as fuck that a friendship that has lasted for about 2 weeks is more meaningful to them than any of the other relationships they have with people because no one else sees them as more than what they are on the surface level.
Honestly I see both Steph and Pete as both deeply lonely people. Sure, they have friendships, but I don't see them as particularly deep or healthy ones. Richie and Ruth are constantly making fun of Pete and crossing his boundaries. The friendships Steph has seem pretty surface level with them also being pretty judgmental of her and her choices. They don't have a lot of people in their corner. Of course they'd cherish the first relationship that gave them a true connection. Of course they'd want to hold onto it for dear life. No matter how short they'd known each other, they made an impact on each others lives in a way no one else had been able too.