Even though she was a grown woman, she believed in elves and Santa Claus and flying reindeer and sacks with an endless capacity for toys. ...

she believed in elves and Santa Claus

Even though she was a grown woman, she believed in elves and Santa Claus and flying reindeer and sacks with an endless capacity for toys.
Furthermore, she believed that believing in these things was the chief reason that she had more fun in life than others.