They said it was a costume party
Don’t you hate it when your friends have no holiday spirit whatsoever, and they find costumes childish? In this case, this guy was told he was attending a costume party, and he so he brought his holiday spirit. To his dismay, he was the only one.
Some people in a similar situation would simply leave, embarrassed to their core. Not this guy, however, who was confident enough of his nostalgic Teenage Mutant Ninja Turtles costume, to stay at the party and have a good time. So what if he’s the only one wearing a costume? He’s about to let this silly mistake ruin his holiday spirit.