The biggest reason for the seasons is that Earth's axis is tilted relative to its orbital plane. Think of the orbital plane of the solar system as a flat plate. Most of the planets orbit around the Sun on the "surface" of the plate. Rather than having their north and south poles point directly perpendicular to the plate, most planets have their poles at a slant. This is particularly true of Earth, whose poles are tilted 23.5 degrees.

Earth may have a tilt because of a large impact on our planet history that likely caused the craetion of moon. During that event, infant Earth was smacked pretty heavily by a Mars-sized impactor. That caused it to tip over on its side for a while until the system settled down.