I'm not gonna defend all the dramatics, because I also felt it wasn't a great film in that regard. Maybe pushing it to the end would have helped give Richard's story some form.
But I think as it is, it had to do with Nat taking him to her house and even forcing the confrontation - remember, Richard chickened out there and wanted to leave. They were helping each other move forward. Did Richard actually move forward? Probably not, in the end. Even though Richard blurting out "I love you" and angrily insulting her family wasn't the right move, Nat did get him literally to the point where he could bury these imaginary feelings that were his big hang-up.
And it does make an impact as it leads directly to him proposing to the other girl, who clearly is just a replacement obsession. (Yes, there's no real growth to Richard's arc, but it is the progression of his story. I think they're related occurrences.)