Shame
This movie portrays sex addiction perfectly, because it has its rawness and boldness that was required for the theme. Both, Brandon & Silly, are such interesting characters and both of the actors did a great job at showing their characters' complex nature.
Safe to say that Fassbender ate this role up and might be one of the best performances I've ever seen in my life. Brandon has no control over his life and his outside world, his work, his house, his every day life is just as empty as he is inside. But it's not just that he's empty, he's also completely ruined. The addiction is what makes him empty, creates the void, makes him feel all these emotions: guilt, shame, remorse, hopelessness, powerlessness, loneliness; makes him depressed and anxious, but even though hypersexuality is what causes this, it's also the only thing that stops it. Funny thing about addictions, for example drug or alcohol, is that they only work for a moment, and when the effect is gone, when you're not drunk or high anymore, your one moment of relief is gone too. It's exactly the same for sex addicts too. Without that addiction, there's no drive for him, no point in life, nothing to hold on to.
Fassbender's performance was so impressive, because it's one thing to portray character and that void well and it's other, when you do it so good, that you can make the audience feel it too. (Or it's just that I'm depressed too and that shit was too relatable)
Speaking of void, there's this phenomenon, "call of void" which is exactly what that scene was showing, when Silly was standing too close to the edge in the subway. Main difference between Brandon and Silly is that, Silly is not that empty. You could say that Silly is full, even. The moment when she was singing, i think Brandon felt that too. Silly's voice was so full of something that Brandon didn't have.