What did Mrs. Bennet find promising about the Meryton Ball for Jane?

Mrs. Bennet found the fact that Bingley danced with Jane twice a promising sign.

“Oh! my dear Mr. Bennet,” as she entered the room, “we have had a most delightful evening, a most excellent ball. I wish you had been there. Jane was so admired, nothing could be like it. Everybody said how well she looked; and Mr. Bingley thought her quite beautiful, and danced with her twice! Only think of that, my dear; he actually danced with her twice! and she was the only creature in the room that he asked a second time.