Sulley: You don't study scaring, you just do it.
Mike Wazowski: You don't think I'm scary.
Sulley: You're not even in the same league with me.
Dean Hardscrabble: I'm here to make good scarers great, not make mediocre scarers less mediocre.
Young Mike: I found a nickel! I wish I had pockets.
Mike Wazowski: I've been waiting for this my whole life! I'm gonna be a scarer!
Answer: They said he could rejoin the fraternity as long as he proved he could be a great scarer, so his victory showed that he was good enough for them.