Because he was born on December 24th, and his parents named him Santa.
Rough break.
She collapses with laughter.
But no, she says, okay, it was kind of funny, but she wants to make him happy.
He says to leave him alone, he's perfectly happy the way he is.
This is a lie.
Now, she has Santa magic, which allows her to create anything starting with "San-". A montage of untranslatable jokes follows, where she misunderstands what he's doing and attempts to improve it by materializing some ridiculous object.
Shenanigans ensue.