Why do we have the seasons?

Because the earth rotates about an axis that is tilted at 23.5 degrees.
Why does the earth’s axis tilt this way? Because we have a relatively large moon (1/10 size of earth) and the gravitational lock between the bodies holds the axis in place.
Mars e.g. which has two very small moons has dramatically unstable axis and so the weather on mars is very unstable.

Derrived from this lecture.