what is the reason for the season change on earth

We observe season on Earth due to the fact that the Earth is tilted on its axis with respect to the axis of rotation. It us such that different parts of the Earth receive varying amounts of daily sunlight during different parts of the year. Thus, sunlight is an important factor driving seasonal changes.

In northern hemisphere, in summers we receive more sunlight compared to winter months. In the southern hemisphere the seasons are totally reversed.