The primary reason for the seasons changing is the tilt of the Earth's axis. The Earth is tilted at an angle of approximately 23.5 degrees as it orbits around the sun. This tilt causes different parts of the Earth to receive varying amounts of sunlight at different times of the year. When the Northern Hemisphere is tilted towards the sun, it experiences summer, while the Southern Hemisphere experiences winter, and vice versa. As the Earth continues on its orbit, regions shift in sunlight exposure, leading to the cyclical changes of seasons experienced on our planet.