Throughout the movie Greenberg talks about "writing a letter" when coming across less than pleasing situations, situations that wouldn't even bring a thought to the average persons mind. Towards the end of the movie we see him mailing letters to a mish-mash of vendors from Starbucks to the local vet. One of his letters gets published in the Times. It's these letters that keep him from going insane which we know he already has.
Roger Greenberg used to be a musician and broke up the band for the same reason that he writes his letters, because things aren't right. He is now a carpenter which seems like a great job. He comes to LA to take care of his brother's dog and "do nothing." But by trying to "do nothing" he seems to fall in love with his brother's house keeper played by Greta Gerwig. Stiller's character is one of those people that can never be happy with anything. He knows that he's in love with Florence, Gerwig's character but he tells his friend, Ivan, that he doesn't want to pursue it because it's easy.
In Greenberg we see Roger go through the movie and we're not sure if he learns anything. In the end he makes a rash decision and ends up not going through with it because it isn't right. It's his general unhappiness with the situations he puts himself in that ends up saving him from making a bad decision but it's also this attitude that keeps him from being happy.
I like this movie and actually was surprised by it. Stiller is good in a not straight up comedy role. I found myself laughing more than Michelle did but I think we both enjoyed the movie and probably for different aspects.