Seasons occur because Earth tilts 23.5 degrees on its axis. That tilt affects the intensity of the sunlight hitting the surface, and that in turn causes the seasons. When North America, Europe, and northern Asia are having summer, it's because they're tilted directly toward the sun and receiving the most sunlight.
The answer is the Earth is tilted as the Earth makes its journey for the year around the Sun.
In general the earth is tilted at angle of 23.5 degrees from vertical axis. So that the Earth always pointing to one side as it revolves around the Sun. This causes varying amounts of Sun light around the Earth during the years gives us seasons.
So here we conclude that the axis of the Earth is most important thing, why the seasons occur.
Tilt:
Whenever the Earth lean towards the Sun, summer season occurs and lean away from the Sun winter season occurs. In between these two spring and Autumn occurs.
