Stanley was a boring fellow, as the Narrator calls him. Stanley has a job which demands nothing of him, and every buttons he pushes is a reminder of the inconsequence nature of his existence. Look at him, pushing buttons, doing exactly as he was told. Now, Stanley's pushing buttons. Now he's eating lunch. Now he's going home. Now he's coming back to work. The Narrator said one might simply feel sorry for Stanley, except that Stanley had chosen to accept his life.
However, in Stanley's mind, he could go on fantastic adventures on unknown new lands, although he reminds himself, that none of those imaginations would ever happen to him each time he returns to work. Stanley imagined himself where one day, he step out of his office to find his boss, his co-workers and every human in the building has disappeared mysteriously from the face of the earth. This imagination excited Stanley terribly.
Then, Stanley imagined, that he will come across two open doors, that'd lead him on many different places. Down one path is a Mind Control Facility located deep down in the building, down the second path is a yellow line, that weaves everywhere. Another path has many games instead. And what does Stanley called it? The Stanley Parable.
This grand imagination of Stanley makes him enjoy repeating it over and over, hoping that if he walked down another path, he'd find a new story. This paradox inside him makes him feels so free.
In reality, all Stanley does is pushing buttons, nothing had changed; the more he imagines his Parable, the more Stanley forgets, which life is the real one. The Narrator claims Stanley can only be an observer. As long as Stanley stays there, every second he spends in his office, he's actually slowly killing himself. The Narrator admits he is unable to make Stanley look at himself, not in the way the Narrator intended.