According to the narrative in Chapter 21 of the book, how does Penny arrive at the conclusion that it will be alright for Mr. Mulligan to marry her mother?

According to the narrative in Chapter 21 of the book, Penny arrives at the conclusion that it will be alright for Mr. Mulligan to marry her mother because she has noticed her mother to be in a happier mood since Mr. Mulligan has become a part of her life.