What is the irony in 'Dead Stars' by Paz Marquez Benitez?

In 'Dead Stars" Alfredo falls in love with Julia when the passion between him and his fiance, Esperanza, has faded. He reluctantly decides to let her go and marries Esperanza and they start a family. The irony is that eight years later when he visits Julia during a business trip he realizes that his love and passion for her has faded away.