Why does Stilton think Bertie grew his moustache? Who does Stilton blame for the moustache?

In Chapter 4, Stilton accuses Bertie of growing his moustache to appeal to Florence and threatens to break Bertie's neck. Stilton also accuses Jeeves of creating Bertie's plan to win Florence, which Bertie argues couldn't be true because Jeeves disapproves of his facial hair.