their friendship is so beautiful, why would anyone want to change that when they're so perfectly platonic and their dynamic is spectacular and there's no awkwardness between them, they are obviously BFFs for life no question
You can see the moment during his confession/plea when you can tell Benvolio starts to get uncomfortable with admitting how much he actually likes Rosaline, how much of a good person he thinks she is, because he defaults to sarcasm and jokes.
“One thing I’ve learned from being stuck with you” And Rosaline rolls her eyes and her entire posture on the balcony changes. She starts to pull away, to close up again, because of course he would make some quip, why had she thought for even a minute that he was taking this seriously, why had she let herself believe that maybe he-
“What I’ve learned from spending time with you” And her eyes flick back towards him and he takes the hint. He takes it and runs with it, really. He starts to expound on her, on what a good person she is; her decency and her honesty and he keeps it barely skimming the surface because going any deeper might scare her off again, but he can’t help but emphasize it - “which in this city is a rare thing indeed” - and you can hear the emotion and conviction in his voice.
He tells her openly and honestly what a good person he thinks she is, and then offers her a chance to come with him and solve this mystery that is driving their families even further apart; to maybe end this feud once and for all and save more people from suffering the fate that her family did, but most importantly, he leaves the final decision up to her. He makes his case and he’s desperate, but he leaves it at asking her one more time, and the last thing he says to her is her name. and i could make a completely separate ranting post about that He ends with her name and leaves her free to decide for herself and I’m like 90% sure that’s why Ros agrees to come with him, because his being open and honest and vulnerable made her confront her own feelings towards him, and the freedom to make her own decision made her finally say yes.