The main cause of variation in seasons on planet earth is the tilt of its axis as it orbits the sun. The tilt of the earth's axis is at an angle of approximately 23.5 degrees from the plane of its orbit around the sun. This tilt causes different parts of the earth to receive different amounts of sunlight at different times of the year.
As the earth orbits the sun, its axial tilt causes the northern hemisphere to lean towards the sun during the summer months and away from it during the winter months, while the southern hemisphere experiences the opposite. This causes the variation in temperature and the change in weather patterns that we experience during the different seasons. Additionally, the earth's revolution around the sun is not a perfect circle but an ellipse, which also affects the amount of sunlight that different parts of the earth receive throughout the year.