Once upon a time a great Saint Durvasa while going out for a walk met dev indra on the way. He gave Indra a garland. However, Indra ignored the saint and threw the garland and went off in his vehicle airavata. On this behavior durvasa got angry, and cursed him by saying that Lakshmi would leave him. Indra realized his mistake and left ashamed. He said sorry to him. Durvasa then said that only Lord Vishnu can help him. Goddess Lakshmi indeed left Indra and Indra lost everything, including his kingdom. Devils attacked Indra. He went to lord Vishnu with his teacher and guide Brihaspati Lord. Vishnu adviced him to make the devils his friends and both should churn the the sea of milk to get the mil of immorality. On churning process, the nectar was produced and drunked by all the gods. Devi Lakshmi immediately appeared there in a beautiful lotus. She gave the pot with nectar to Indra and the devas and Indra got back everything that he had lost earlier.
This is the story of dev indra and devi lakshmi and how Laxmi was born to restore wealth. Thus Diwali festival is celebrated to commemorate the birth of the Goddess of Wealth.