"Winter" is in many ways a nature poem; there are owls and birds and descriptions of winter scenes (icicles, snow). But it's not just about some frozen forest, but about what people do in this winter wonderland. It's about how humans make their way through the bleakest time of the year. Yeah, life's basic necessities (milk, water, blood) are frozen or chilled, but people manage to overcome nature's lack of hospitality and soldier on.
The natural world is man's friend and enemy—his frenemy. In this poem's tough winter, the forest furnishes apples and logs for survival.
At one with nature? Puh-lease. While humans can learn to adapt to nature, or deal with it, man and nature will always be fundamentally separate.
If spring is all about life (new birds are born, flowers bloom, the weather gets nicer), winter is all about death. Note all the images of death, or semi-death, in this poem: frozen things (icicles, milk), cold things (blood, snow), owls (they hunt), etc. Sure, "Winter" isn't all about actual death and dying, but the lifelessness and coldness of winter cannot help but make us think of death. Cheery, right?
Life1, Death 0. Although death is everywhere, life always manages to come out on top in "Winter."
Death is scary and all that, but it's also a natural part of life. The owls, for example, need to eat. Sorry about that, little mouse.
If one part of "Winter" is the desolate, winter landscape, the other part is some nice little cottage where various homely activities are taking place (cue the holiday music). "Greasy Joan" is stirring a pot of something (probably stew or soup), Dick is carrying logs (for a fire in the hall), and there are crab apples in a bowl of ale. Despite a seriously cold and rough winter outside, the "home" is doing just fine. In fact, it's the rough weather outside that makes the home-y inside so much more inviting.
Let it snow, let it snow, let it snow. The warm, inviting, homely atmosphere subtly described in this poem wouldn't be the same if it weren't snowing and freezing outside.
Home is where the heart beats. The home is the primary symbol of life in this poem of death.