Final answer:
Sir Gawain's change of heart towards Sir Launcelot in 'Le Morte d'Arthur' is caused by a prophetic dream, in which he foresees the downfall of the Round Table and Arthur's death due to the persisting enmity.
Step-by-step explanation:
In Sir Thomas Malory's Le Morte d'Arthur, Sir Gawain undergoes a change of heart regarding his feelings towards Sir Launcelot due to a prophetic dream. Gawain, who was fatally injured in a battle, dreams that his hatred for Launcelot will bring about the downfall of the Round Table and cause the death of King Arthur.
Realizing the error of his ways, Gawain writes to Launcelot to ask for forgiveness and to return to England to aid Arthur, signifying Gawain's reconciliation with Launcelot. This act represents a deep moral and emotional transformation for Gawain, who until that point had been one of Launcelot's staunchest opponents.