There's multiple reasons for the changes of seasons on the Earth, the most important being the elliptical orbit of the Earth around the Sun, its inclination, and the angle by which the Sun's rays are falling on the Earth.
Because the Earth's orbit is elliptical, in certain parts of the year it is closer to the Sun, and in certain parts of the year it is further from the Sun. When the Earth is closer to the Sun, it speed increases slightly, and that makes the Earth cooler, and when it is further from Sun, its speed decreases by little, and the Earth is warmer.
The inclination of the year allows for one half of the planet to get more Sun light in one part of the year, and the other half to get less, while in the other part of the year, the half that was getting less Sun light gets more, while the one that was getting more is getting less.
The angle of the Sun's rays is directly connected to the inclination of the Earth and its shape. The parts that get the Sun's rays at an angle of 90 degrees or close to that are warmer, and as the angle is becoming smaller and smaller the parts of the Earth become colder and colder because the rays are dispersing over a much larger area.
