Finn had fallen in love with Rachel because of the way she sang with all her heart and soul. She would pour her voice into every word, making the lyrics fit around her emotions. That was why Finn was looking forward to Rachel's solo at regionals- he knew she would be beautiful. It was going to be a surprise for him- he hadn't heard more than the chorus, and that was fantastic as it is.
Backstage, Finn pulled out of Quinn's arms to watch Rachel's performance. She looked tiny, almost childlike as she walked to the pink microphone and began her performance. She started small, but the force built behind her words until they burst at the chorus when Tina and Brittany began the background choir. Rachel's eyes were streaming with tears, and Finn had to look away- had he really caused all of that? Did he really have so much of a role in her life? He realized that from the very beginning, Rachel was his best chance at becoming better than the jock, the almost-bully. He tore himself away from his thoughts to listen to Rachel, confidently taking the microphone in her hand to command the stage. She pumps a delicate fist in the air, so strong and yet so fragile- it makes Finn tear up a little. And then… Rachel does it. She hits the bridge and her voice soars throughout the auditorium like an eagle, honest and brave. He doesn't know how she manages but she holds the note beautifully, and all he can think is, (oddly enough) Quinn could never do that. No, Quinn's voice starts off nicely, and then fades away like footprints in the sand… She could never mark her voice on Finn's heart like Rachel had… Never ever. And it is then he knows he needs to leave Quinn, leave her as fast as he can because she was toxic- poison because he's acting like the douchebag from last year, and that's all her influence. As Rachel thanks the audience Finn feels his heart racing, not from nervousness but from anticipation. He'd won Rachel once; he'd do it again, and this time, he'd get it right.