The apostle James uses the strong words "hatred" and "enemy" to drive home the point that we can be in the world or in the kingdom, but not both because they are at opposite ends of the spectrum. Those who choose worldliness choose to live in the enemy's camp because all that is of the world is under the control of Satan (1 John 5:19). He is the ruler of this world, and when we choose the world, we enlist in his evil army and become enemies of God. For the Christian, the choice is clear. To avoid worldliness, we must mature in the faith, growing up in all things in Christ so that we are no longer spiritual infants, tossed about by the lies of the world (Ephesians 4:14-15). We must come to know the difference between the wisdom of God and the foolishness of worldly wisdom, Read more: http://www.gotquestions.org/worldliness.html#ixzz2cLaA70rc
At James 4:1-3, we read: "From what source are there wars and from what source are there fights among you? Are they not from this source, namely, from your cravings for sensual pleasure that carry on a conflict in your members? You desire, and yet you do not have. You go on murdering and coveting, and yet you are not able to obtain. You go on fighting and waging war. You do not have because of your not asking. You do ask, and yet you do not receive, because you are asking for a wrong purpose, that you may expend it upon your cravings for sensual pleasure." It was after writing these words that James warned against friendship with the world. The Christians of that time did not murder one another or kill one another literally, that happened centuries after. James apparently had in mind a failure to love fellow Christians. (Titus 3:9; 2 Timothy 2:14; 1 John 3:15-17) Covetousness and "cravings for sensual pleasure," pride, jealousy, and ambition rupture the loving Christian fellowship in a congregation. Such attitudes make one a friend of the world and, hence, an enemy of God. None who harbor attitudes like these could expect to remain a part of God's organization.
James 4:4 " You are like an unfaithful wife who loves her husband's enemies---the evil pleasures of this world--makes you an enemy of God? I say it again, that if your aim is to enjoy the evil pleasure of the unsaved world, you cannot also be a friend of God." Living Bible. That is very clear as well. Careful as to who you listen to, "Now the Spirit expressly says that in later times some will depart from the faith by giving heed to deceitful spirits and doctrines of demons, through the pretensions of liars whose consciences are seared, who forbid marriage and enjoin abstinence from foods which God created to be received with thanksgiving by those who believe and know the truth." ( 1 Timothy 4:1-3. Revised Standard Bible
"Friends with the world"means you are more interested in giving yourself to the wicked endeavors that the world wants to lure you into, and that the more time you spend with the "world" the farther away from God you are getting.