The seasons change due to the orbit of the earth being an ellipse instead of a perfect circle and the inclination of the earth's axis to this orbit. Earths axis changes and rotates because earth is at a tilt that means in different places of the earth there are different amounts of sunlight that will hit the surface. Sunlight means heat and energy. So if you live in the northern hemisphere that means the tilt of the earth makes the northern hemisphere face more up and sunlight is harder to reach up because the earth is at a 23 degrees axis, as it orbits the sun part of the time the northern hemisphere is bathed in more direct sunlight and the other part the southern hemisphere. The earth is slightly tilted on its axis. As the sun shines on the earth, it shines more directly on the northern hemisphere in June (summer), and more directly on the southern hemisphere in December (winter). That's why the seasons are different.