Why does Jack praise the bread and butter Algernon offers him?

The reason for Jack's praise never truly becomes evident, though we can infer that Jack's reaction is sarcastic. Jack wanted the cucumber sandwich, but Algernon refused him, as he's ordered the sandwiches specifically for Gwendolen. Jack, on the other, hand praises what he's given, possible to make Algernon feel foolish, or possibly because the bread and butter was truly enjoyed.