What is the single most important factor that determines why Earth (or almost any planet in the Solar System) experiences seasons?

Planetary seasons are caused by two factors:

1. axial tilt and

2. variable distance from the sun

Explanation:

Earth's orbit is nearly circular and so has little effect on climate. It's our planet's axial tilt that causes almost all seasonal changes.

You can read further; Planetary seasons

it is called kinetic energy

energy can be converted from one form to another, but it cannot be created or destroyed in ordinary chemical or physical means. do not understand what you are asking but hope this