The part that goes "watch the worlds collide" in I Didn't Understand is heartbreakingly beautiful if you compare both versions and knew the story behind it

He wrote one version for the girl he's breaking up with and another for the girl he wishes he's been with. To me "watch the worlds collide" mirrors his feelings about two women. It's like he's torn between leaving one person and loving another. And for us the listener we can compare the two versions and "watch the worlds collide". I sound corny but that's what I felt listening to both songs