Although most artistic movements are born as a reaction against aesthetic forces of their day, Romanticism and Impressionism were reactions against a codified idea system. Born in response to the Enlightenment and its application of rational, natural law to science and nature, Romanticism, according to WebMuseum, retreated to a nearly mystical reverence for nature and the natural world; rather than trying to understand it, romanticists merely wanted to enjoy the world around them. Impressionism was an even more openly rebellious school of painting; painters cast off many of the fine-art teachings of the day and instead focused on capturing light and emotion in their subjects.
Romanticism was built around a reverence for nature, and many of its principal works glorify the natural world through the use of realistically rendered scenes composed to inject a sense of fascination and awe into the viewer. Although Impressionism wasn't as neatly tied to the natural world as Romanticism was, many of its painters concentrated on the effects of light, particularly sunlight, on their subjects. Sometimes using a time-lapse approach, Impressionists, as Hamlin Garland noted, drew much inspiration from the way subtle changes in natural light---seasons, atmospheric conditions, time of day---could drastically alter the mood of a painting.
Whereas earlier, more realistic painting styles were concerned with accurately rendering their subjects on canvas, Romanticism and Impressionism were more focused on creating a painting that captured the spirit of its subject. For Romanticists, that meant a realistic rendering of a scene chosen and composed specifically to create an emotional resonance in viewers that pulled them away from the scientific and rational---romanticists would say clinical---approach to rendering a subject. Impressionists took this idea even further, opting to discard many of the fine-art considerations of realism and delicate brushwork in favor of canvases that captured the spirit of the moment, using visible brush strokes and lighting effects that discounted realism to control viewers' emotions.