The primary reason for the seasons on Earth is the tilt of its axis as it orbits the Sun. This axial tilt causes different parts of the Earth to receive varying amounts of sunlight throughout the year. Consequently, areas tilted towards the Sun experience warmer temperatures and longer days, leading to summer, while areas tilted away experience cooler temperatures and shorter days, resulting in winter. The phenomenon of varying solar angles affects not just temperature but also the length of daylight, creating the distinct seasonal changes we observe.