When Will first meets Maguire, he walks right up to a painting on his wall and finds out that Maguire painted it himself. He then uses his knowledge of psychology and art to make claims about Maguire's life based on the painting. For starters, he says,

WILL: […] he linear and impressionistic mix makes a very muddled composition. It's also a Winslow Homer rip-off, except you got Whitey, uh, rowing the boat there.

So for starters, he calls Maguire out on being unoriginal. But his mind games don't stop there…

The longer Will looks at the painting, the more he seems to figure out about Maguire's life. By the time he's done, he's summing up Maguire's whole mentality, saying,

WILL: You just piss in your pants, you're cryin' for the harbors, and maybe you do what you gotta do to get out. Yeah, maybe you became a psychologist.

This is all incredibly insulting and Maguire handles it well. But the moment Will mentions his dead wife, he snaps and grabs him by the throat. It looks like Will has hit a soft spot.

So far, all of this conversation about the painting has been about Maguire. But in a later scene, Maguire turns the whole thing around and makes it about Will, saying,

MAGUIRE: [You] presume to know everything about me because you saw a painting of mine and ripped my f***in' life apart.

The fact is that Will might think he knows people, but he doesn't know anything that Maguire couldn't go find in a book. Will is just a scared little kid who uses his intellect as a weapon to keep people away from him, and it takes Maguire less than a day to figure this out.