In "Araby" by James Joyce, what does Mangan's sister do to make a trip to the bazaar so important to the narrator?

Really, the only thing Mangan's sister does to make going to the bazaar so important to the narrator is to speak to him about it.  She had never actually spoken to the narrator before, and so, when "At last she spoke to [him]," he becomes confused and flustered.  She tells him that it will be a "splendid bazaar" and that "she would love to go"; however, she cannot because she'll be on a retreat with her convent school.  This is enough.  Mangan's sister has spoken to the narrator, and upon this subject, expressing her desire to go to the Araby bazaar and her regret that she cannot.  He seems to imagine this as some kind of quest: he can go in her place and bring her something that will make her happy, and this will make him happy.  In the days between his vow and the bazaar, the narrator finds that he cannot even concentrate on "the serious work of life" because it now seems like "child's play" in comparison to the thing that he desires.