@HEXyren: It's not that hard: MC and FMC were a couple in high school and bound to a career in music together. Problem: she's a genius who'd go far, while he's just average. Before dragging her down and robbing her of her dream, he broke up with her. She took it badly (who wouldn't?) and think of him as a coward. They separated and went with their lives; she became a successful singer while he got diagnosed a terminal disease that will kill him in 6 months. He chose to tell nobody about this.
Now, it appears everyone wants them to make up despite nobody but the MC knowing the full, dramatic picture.