Refers to a cheerful, positive outlook on life; often used in phrases like 楽天家 (optimist) or 楽天的 (optimistic).
He never loses his optimism no matter what.
They say optimists live longer.
悲観 means pessimism, the direct opposite outlook to 楽天's optimism.
Compound of 楽 (ease, comfort) and 天 (heaven, nature), suggesting a temperament that takes things as they come, trusting in a benevolent natural order.