Why does earth have seasons

The tilt of the earth on its axis. When it is tiled one way, the sun hits more directly. When it is tilted the other way, it is more spread out.

In the summer season, Earth's northern hemisphere is tilted towards the sun and in winter season the northern hemisphere is away from the sun. The tilt actually causes a lot of difference in the amount of sunlight received by the earth and since the sunlight is the main cause of the weather change that leads to different seasons.