One way to understand Romanticism is to contrast it with realism. If realism tries to depict the world the way it is, warts and all, Romanticism tries to depict the world the way we would like it to be, heightened in ways that bring out its beauty and enchantment.
Romantic authors focused on finding the divine in nature and the best in the common person, often elevating despised groups, such as peasants or gypsies, to reveal their humanity. They embraced the magical and supernatural rather than emphasizing the merely prosaic. They tried to depict a world alive with a divine light where people lived up to their best selves. They also wrote lyrical poetry—poetry that expressed personal emotion rather than, say, celebrating the life of a great hero.
Keats fits the definition of a Romantic poet. He exalts nature in poems such as "Ode to Nightingale" and "Ode to Autumn." In "Ode on A Grecian Urn," he pours out his emotional desire to be one of the figures on a Grecian urn living in an eternal spring, eternally young and eternally in love. He is unabashed in expressing his happiness as he contemplates what such a fate would be like. In poems like "The Eve of St. Agnes," Keats explores the medieval, folkloric, and supernatural—all important aspects of Romanticism.
In his celebration of nature, personal feelings, and poetry about the supernatural, Keats is the quintessential Romantic poet, transporting us to worlds that are not our own. At times he is melancholic about the world and human mortality, but his poems nevertheless do not dwell on the sordid side of life.