Nikki and Cara, although it happens in one long scene, there's a month or 2 that goes in that flashback... 2 months sharing a house, seeing each other everyday, helping them...
Cara in particular kissed the MC at her birthday party, before he moved in, so something was there from the start.
Nikki, things in common, working out together, saving her from getting ran over, helping with her ex... and it clicked.
Layla is fine, she's the young and innocent archetype getting the hots for a boy... and despite that fact that she's flirty right away, it took her a long time to commit, nothing wrong with that.
Agreed on Erynn, she's obviously predisposed to like the guy due previous relationship, that brings positives and negatives.
Positive that she's always gonna pay attention to him and give him time, and wants him close to her by default.
Negative that she's a little(very slow actually) slow moving because she's scared it might hurt the friendship.
That, plus her history of abuse, makes her both an easy target... and slow to commit.
The way things play out make sense.
Alexis is a bit harder for me to buy too, he stole a kiss from you 5 years ago, he looked way too young for her back then, and after that you never saw him again...
She's not some innocent virgin like Layla and Cara, so it's a much harder sell that she's immediately obsessed with the guy.
Valeria is the "realistic" hot girl, she has a bunch of dudes going after her so isn't dropping head over heels for anybody.
She knows the power that she has and plays with her cards close to her (awesome
)chest and keeps her options open.