Summer is hot because Earth swings closer to the Sun, and winter is cold because it drifts further away. It is intuitive, it matches the campfire logic of step closer, get warmer, and it was confidently given as the answer by most of a graduating Harvard class in a famous education film. If Harvard grads said it, you are in good company.
It is the tilt. Earth's axis leans about 23.5 degrees, and as the planet orbits, each hemisphere takes turns leaning toward the Sun. The hemisphere tilted sunward gets more direct, concentrated light and longer days, which is summer; the other hemisphere gets glancing light and short days, which is winter. The clincher is that the seasons are opposite in the north and south at the same time. When it is summer in Australia it is winter in Europe, which is impossible if distance were the cause, since the whole planet is the same distance from the Sun on any given day. In fact Earth is closest to the Sun in early January, during the southern summer and the northern winter. Distance barely moves the needle. The lean does all the work.