- extremely: to the utmost or to an exceedingly great degree
- incredibly: so extraordinary in quality or kind as to seem impossible
There are multiple reasons for why Foer chose "extremely, close, incredibly, loud" as key words in this novel. The first reason is that the premise of the plot is built upon the explosions of the World Trade Towers on September 11th of 2001. For workers and residents in New York on that day, the explosions were extremely loud and incredibly close. People on the streets and on their residence balcony's found the noise of the explosions deafening because they were so loud and the sight of the explosive fireballs blinding blinding they were so close. This is fact. Therefore it must be a fact in the premise of the story.
Here are some of the first instances in which "extremely" and "incredibly" occur in the narrative text indicating thematic reason for their use:
- incredibly angry
- extremely long limousine
- incredibly long limousine
- Reconnaissance Expeditions were extremely simple
- incredibly complicated
- extremely precise lines
- incredibly bad eyes
Another reason is that the explosion--blinding and deafening for survivors--was part of his Dad's experience, and Oskar cannot rest or find personal peace until he knows the details of that experience. Therefore what was so loud and so close becomes the driving force behind Oskar's actions. This is accentuated because Oskar is not extremely, incredibly close to his mother and believes (reliably? unreliably?) that his mother would have traded his life for his father's:
I could tell that she didn't really love me. I knew the truth, which was that if she could have chosen, it would have been my funeral we were driving to.
Another reason is that "extremely loud and incredibly close" is a metaphor for Oskar's relationship with his Dad. That relationship is now supplanted (replaced) by the other thing--the loud close event of 9/11--that is now as incredibly close to his psyche and experience as he used to be to his Dad. This new thing that was and continues to be extremely loud and incredibly close has become the driving psychological motivation behind Oskar's characterization.
Being with [Dad] made my quiet. I didn't have to invent a thing. ... I tucked my body incredibly close into his.