James 4:4

So fights break out among them who have their passions waging war within them. Their desires are not fulfilled, so they resort to murder and fighting. They don't have because they don't ask, and they don't receive because they ask incorrectly. Attempts to spend these things on their passions mean friendship with the world. Instead of being a friend of God and asking it from him and not desiring to spend it on passions, befriending the world is what people resort to. This is so bad that James calls them adulterous. It is adultery because the bible describes us as being wed to Christ, and by renouncing that and going to be with the world is adultery. You can't be both friend of God and friend of the world; you are either one or the other. As a result, choosing friendship with the world means being an enemy of God.