It seems like lots of things in Twilight can be sorted into two opposing categories, all based on one major dichotomy: humans versus vampires. Bella (representing humans) states on the first page of the book that she prefers her warm, dry, sunny Phoenix to cold, rainy, cloudy Forks. Guess who's cold all the time? Ding ding! Edward (representing vampires). There's a big divide between the two species, and it seems that humans are mostly identified with "good" things, while vampire has lots of "bad" or "evil" things on its side.
Then there are a few confusing moments in the novel, when it seems that a person or two might be switching sides. There's the scene with Bella and Edward in the meadow – when Edward reveals his sparkling ability – that contradicts lots of the "cold" and "dark" associations we have with his character. He does like the sun, the light, the warmth – he just can't take advantage of it around people who don't understand his "species." Remember, the Cullen home is very light and airy; it's the "one place [the Cullens] don't have to hide" (15.212). Like the other Cullens, Edward would indeed rather not be a "monster" – hence their no-human diets. That doesn't change the fact that they have to live in the land of dark, wet, rainy, cloudy, cold Forks in order to have a relatively normal life.
On the other side of the dichotomy, Bella's initial move to Forks could symbolize her inching toward the dark side of the spectrum – except she vocally hates it. Over time, the closer she grows to Edward, the less she hates Forks. By the end of the novel, she's ready to completely transfer to the vampire side, but that's when Edward draws the line and won't let Bella cross it. When the novel ends, Bella still longs for him to transform her. The last line of the novel, "And then the night closed over me," made us think that maybe her changeover is inevitable.