He's got a craving that cannot be fulfilled. It's gnawing at his bones and tearing at his soul. Because he knows that he can't (won't) love Blair. He won't(can't)because he's Chuck Bass and in the script he's just the 's just there to keep the plot a little interesting. He only amounts to a secondary character that will never be with Blair. He'll sit by her side and plot. He'll sit beside her because this is as close to marrying (being) with Blair that he will ever come. They might have a few drunken nights together or a few schemes to bring up at the 20th High School reunion. So he pretends that he isn't interested in Blair because he knows. He knew longer than his father, Nate or Serena knew that he would never quite be enough. He's not enough because Blair wants to be holding hands, kisses on the cheek and a marriage. He knows that if they were together he would wind up somewhere in an island with a Cancun Barbie .So he stands on the sidelines watch Nathaniel looking at Serena with loving and googly eyes while Blair isn't watching. But Blair sees and she knows that she isn't enough. But she'll stay with Nathaniel if it means that the people who are watching won't see.