This is Why was made as a sort of creation story in the form of an epic poem. The explanation is rather confusing; for further understanding please refer to The Long and Messy Family Tree.
In the beginning, there were six; Wind, Fire, Water, Earth, Peace, and Magic (Peace and Magic were originally named Falaes and Korae, respectively). Peace and Magic had a child, whom they named Willow. As Earth grew up, several others came into existence, including Star, Aspen, and Mountain. Earth had two children with Star; Winter and Summer. Summer had a son, Green, with Mountain. Green and Aspen begat Autumn, who fell in love with the child of Fire and Rain (Wind and Water’s child), Spring. Together they begat Man and Woman.
Man and Woman steadily grew greedier and greedier, eating up all the resources of the world, until they each demanded one of the six to eat. Peace and Magic were offered up together; this is why neither exists in the world today.
After his parents’ disappearance, Willow sat down and wept for so long that Earth took pity on him and turned him into a tree; this is why willows weep. Winter was so upset that she beat her mother Earth with her tears and wailing, which is why winter is cold. Mountain was upset by this because he was so cold all the time, and so Summer tried to protect him. She could only do so half the year, however, so Mountain demanded that she make some kind of blanket for him so he could be warmer. The only apparent way to do this was to turn their son, Green, into a tree; this is why evergreens cover mountains. Aspen, Green’s lover, was so upset that she begged Summer to turn her into a tree as well. She is always watching to see if her precious Green once again walks the earth. This is why aspens have eyes.