To have a warm, familiar attitude with this world is to be on good terms with God's enemy. What does it mean, in more practical terms, to be a friend of the world? It is to adopt the world's set of values and wants, to desire what the world wants instead of choosing according to divine standards or divine truths.
In other words, if a person does that, he has actually made himself subject to Satan because Satan is the god of this world! That is a choice that we want to avoid. The worldly person will almost invariably choose to satisfy himself and take action on his desire, which eventually produces confusion, division, and war. It cannot be otherwise because the spirit of the world is the spirit of Satan, and laws are at work that will produce what they are designed to produce.
That was the problem in the congregation to which James wrote. If another apostle had been writing it, such as the apostle Paul did in I Corinthians 3, he would say, "You are yet carnal." These were converted people but still carnal, and they were showing it through their choices. It was not that they did not have the Spirit of God but that they were still so weak spiritually. They were choosing to fall back on what they had in the way of character, understanding, knowledge, and vision from the world, and by this, they showed that Satan was still dominating their lives.
This is understandable because Satan is a wily and powerful adversary—but he can be overcome and defeated. Christ did it, and we can too because Christ is in us.