The way Frank called Dennis "buddy".
The idea of him being able to deal with everything else (somewhat) well but he cannot deal with Dennis. He can deal with everyone holding him at arms length, he can deal with Robby giving him the cold shoulder, he can deal with the guilt that comes with treating the patients he took advantage of but Dennis? Coming face to face with him makes his skin itch. Seeing him wakes up something mean in him, stirs up that dark and ugly part of himself he's been trying to work on for the past ten months.
The satisfaction he got from seeing Dennis still rocking the "student doctor" marker on his badge was petty and juvenile but god he just couldn't help the feeling of pure delight that coursed through him. Couldn't help but make a comment about it just to see if Dennis would squirm and flush with embarrassment.
Is this really the new golden boy? This is my replacement? What a joke.