The Mythology Associated with Deepavali
Mythology has its own reason to offer for the Diwali festival. According to mythology, the celebration of Diwali is closely bound up with one of India's two classic epics the Ramayana. (Mahabharata being the other epic). In brief the Ramayana story is as follows:

Dashratha, goes on a Hunt and Wounds a Human
Dasharatha the king of Ayodhya had once set off to hunt deer. In the midst of the forest he heard the sound of bubbles and saw some movement in the distance. Thinking it to be a deer drinking water, he shot an arrow in that direction.
Much to his surprise, as his arrow found its mark, he heard a human being cry out. When he rushed to the spot his dis-believing eyes saw a handsome youth fatally wounded by his arrow.

Sad as he was, Dashratha took the dying youth, whose name was Shravana to his (the youth's) blind parents, who were nearby. On hearing from Dashratha, the demise of their only son, they cursed him that he too would find himself in the same agonizing state when his son would be snatched away from him.

Dashratha Grants Three Boons to Kaikeyi
Long after this incident faded away, King Dashrathacontinued to reign over Ayodhya. One success followed another and with his three wives and four sons, he was a model of happiness. His first wife bore him Rama, the second one Sumitra bore him Laxman and the last one Kaikeyi bore him Bharat and Shatrughna.
Each son was nobler than the other. Once during a battle, a wheel of Dashratha's chariot came off, Kaikeyi who had accompanied him to the battle inserted her hand into the axle and held the wheel in place.
Her brave gesture enabled Dashratha to win the battle. In return Dashratha granted her three boons and told her to ask for anything she wanted. As she wanted nothing at that moment she reserved the right of asking for their fulfilment for some later date.