I watched it yesterday, so I didn;t just notice it.
I#m a liar. This will contain spoilers:
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Well, my interpretation is that the harmonium is a representation of Barry. He first finds it and instantly puts it inside his office, which is almost completely surrounded by glass.
Now, the story of him as a kid (throwing a hammer through a sliding glass door) and him actually doing it at the birthday party is his means of escape. He feels enclosed, seperated from the world, and by smashing the glass he's trying to escape from the situation he's currently in.

So, back to the harmonium. He puts it in his office, which symbolizes him feeling trapped. It's all broken. The next day, after getting a dangerous call from the sex hot line, he takes the phone with him to work in a hurry and puts it ontop of the harmonium. See, the whole deal of the sex hotline thing is on his shoulders and it's presuring him. He gradually fixes it the further into the relationship he gets with

When Lena walks in the warehouse with Barry's sister, Barry rushes to the harmonium and starts to press a few notes. Obviously, he's trying to keep a hold of himself, pick up his feet and not be nervous. Whenever he gets angry in the office he will hold the harmonium, to keep himself together and to try not to lose it. When he can't use the pudding vouchers for six to eight weeks, he punches the wall then falls onto the harmonium. He now has to rely on himself to get to his lover, and the word love is in dried scars on his hand.

Then, skipping to the end of the film, he sprints into the office, takes the harmonium and runs to Lena. Finally, for the first time in his life, he is free, feels better in the world and feels comfortable. The last shot is of Barry playing the harmonium, it is completely fixed, and he has notes on the top of it, a way of guidance, a sense of somewhere to go in his life, finally diversified. Lena comes behind him and says that famous line "So here we go!" which is obviously an open ending, but probably refers to the future of their relationship.

Yeah. Punch-Drunk Love is fantastic. Full of fantastic cinematogrophy. I could say it's this gneerations Citizen Kane.

What I love most about this film is the character study of Barry Egan. Alike to [i[Taxi Driver[/i], he's lonely and a misfit so to speak. The fantastic scene between him and his sister on the phone in Hawaii was brilliant, him beating the hell out of the four brothers was one of the most satisfying film scenes ever made, and when he confronted Dean... wow.

Everything about this film is utter perfection. Paul-Thomas Anderson done absolutely everything.
I could go into detail about colouring and lighting and how all that is interpreted in my point of view, but I won't.

Oh and, an interesting thing right here. The car crash at the start, many people interpreted as love striking at any time...

"I wanted to start the film out with a bang..."
said director Paul-Thomas Anderson when interviewed and asked about this.
Just goes to show how deep people look.

Oops. I pulled a Bickle. I contradicted myself, damnit.