There wasn't a single part, Madness. It was the way the story was told, the camerawork, lighting and post production work were very strong and as important as any of the four major characters and their individual story arcs. The sum of all these parts treat all the characters as the same, to the point where the montages, etc. started to bleed from one character to another. There was even a sequence where Jarred Letto is fantasizing about his girlfriend and she is wearing the red dress that drove his mother into addiction. Each character was motivated by another fifth unseen character, namely the face of addiction that lies within us all. Each fails to take action to stop their addictive tendencies and chase after things that are unobtainable or fleeting in nature, eventually killing all their dreams, hence the title.