What relationship between the earth and the sun causes the seasons to occur?
The earth's tilt.
The earth tilts at a 23.5 degree angle on the solar plane.
A representation, not to scale, is shown above. The black line which crosses the middle of the sun represents the solar plane. As can be seen, when the northern hemisphere is tilted toward the sun, it is summer there. When the southern hemisphere is titled towards the sun, it is summer there.