We have seasons for the same reason that a stone skips when you throw it close to the water, skipping a rock.
Consider a still pond. Think about the part of the pond close to where you stand-- in other words you are not concerned about somewhere further out in the pond. You have some rocks. You are going to throw the rocks and want to make as many waves in this part of the pond near you as possible.
How will there be more waves right here in front of you?
You could throw the rocks straight up in the air and let them crash down on the water at 90 degrees. Or you could throw them at an angle that is closer to parallel to the water, skipping the rocks across the surface. Well, throwing them straight up in the air will make more waves, of course.
So, now, forget a pond and rocks and think about beams of light coming from the sun. Same deal. The waves are symbols of energy absorption. A straight beam of light hits the earth like the stone tossed in the air crashing down on the pond. A beam at an angle skips across the surface of the earth like the skipped stone.
Thus, winter and summer. Summer is the rocks coming crashing down on the water and making bigger waves and winter is the rocks skipping across the surface.