n the movie, 18-year-old Sulley was shown to be very different than he was later in life.
Being the son of Bill Sullivan, an accomplished Scarer, and having the natural talent that came from his family, Sulley was more laid-back, arrogant, cocky, immature, and something of a party animal. Because of his talent and coming from a family with plenty of accomplished scarers, Sulley wasn't a model student, once stating "You don't need to study Scaring; you just do it!"
He developed a competitive rivalry with a fellow student in the Scaring Program, Mike Wazowski, who had the brains and intelligence for Scaring but not the natural talent. He was also chosen to join the Roar Omega Roar fraternity due to his Scaring talent. To his frustration however, Mike was beating him in class because of Mike's keen studying while Sulley wasn't doing well. Johnny Worthington III temporarily took his jacket unless Sulley passed the exam and not losing to Mike.
Sulley realized just having the talent wasn't enough when Dean Hardscrabble kicked him and Mike out of the Scare Program, for which he blamed Mike (evident by constantly glaring at him and crushing a scream-canister in his hands). Roar Omega Roar also took the jacket away, which meant that he was kicked out of their group. But later, he was desperate enough to join Mike and the Oozma Kappa fraternity to get back in by winning the Scare Games when he overheard Mike and Hardscrabble's wager, that if OK won, the whole team would be allowed in the Scaring Program.
He smugly got himself on the team because Mike had no choice since he needed one more member. He came off as shallow, deeming Oozma Kappa to be lost causes (even suggesting to Mike the get a new team as well as cheating) but eventually came to appreciate them for their friendship. Even so, Sulley wanted so bad to get back into the Scaring Program he rigged the last event so that Mike would easily win, as he honestly didn't think Mike was scary.
At first, Sulley tried to justify his cheating but realized what it almost cost him, especially when Mike sought to prove himself by scaring actual children, and when an argument erupted between them, Sulley revealed that being a Sullivan made everyone expect him to be scary, and while he acted scary and confident, in truth, Sulley was terrified of the expectations he had to live up to.
The friendship between Mike and Sulley enabled them to surprise Dean Hardscrabble, who indirectly encouraged them to work as mailmen for Monsters Inc., where they worked together until at last they achieved their dream of becoming a Scare-Team.