In oil painting why shouldn't you dry your paintings in the dark?

Answer 1

Because this would cause a thin film of oil to rise to the surface, yellowing it.
Sometimes, when oil paintings are dried in the dark, the painting can yellow. This is because a thin layer of oil can come to the surface as it dries. Drying it in daylight will avoid this from happening.

Answer 2

If the paint is made of linseed oil (most oil paints are), the paint will go yellow in the dark. But it's not really bad - if it's put back in the daylight, it will go clear again, even if the paint is already dry.