It's good to offer to buy pizza for your friends.
Xavier's friends just spent all day helping him move into a new apartment, and they ordered pizza since they're starving.
Xavier wants to thank his friends.
Xavier pays all the pizzas in order to show his appreciation for his friends' help.
Xavier gives his friends money for his share of the pizza.
Xavier's friends spend the evening hanging out in his new apartment, enjoying each other's company.
Xavier's friends feel unappreciated and decide to think twice before helping him again.