I'm going to answer this question very basically, which I assume you are looking for since you didn't ask for theories or data.
The basic reasons for the change of seasons are 1.) the tilt of the earth on its axis 2.) the effect that has on the exposure to the sun and the length of days as it orbits the sun. The axial tilt of the earth is 177 degrees and that remains constant. The thing that changes is the position of the earth in its orbit.
When the tilt of either the northern or southern hemisphere is directly toward the sun, extending the length of days and exposure to the sun, it is summer.
When the tilt of either the northern or southern hemisphere is away from the sun, shortening the length of days and exposure to the sun, it is winter.
Spring and Fall occur when the earth is in its lateral positions.