The primary reason for seasons on Earth is its axial tilt and orbit around the Sun. As Earth revolves, different hemispheres receive varying amounts of sunlight, resulting in seasonal changes. When the Northern Hemisphere is tilted towards the Sun, it experiences summer, while the Southern Hemisphere experiences winter, and vice versa. This tilt ensures that areas of Earth undergo a cyclical pattern of temperature and daylength, leading to the distinct seasons we observe throughout the year.