If the world hates you, it is because it hated me first, since no servant is greater than the master. It would appear rather strange that a few verses after telling the disciples that they were no longer “slaves” but “friends,” Jesus goes back to the “slave” language. A closer look at the context and a better understanding of terminologies will help us understand why. A slave/servant is subservient (defers) to the master, and is always in the service of the master’s interests. A (true) friend, on the other hand, is not subservient but rather comes across not only as an equal partner but also as one who has the interest of the friend/partner at heart. Jesus told his disciples that he had made them friends because he had revealed to them the secrets of his Father’s household. But even as friends, they were bound by the rules of engagement. Their friendship was hinged upon them ob-serving (putting to practice) Jesus’ command to love. In other words, even as friends, they were still required to do something. And this doing of something takes place in the name of Jesus, their friend and master. 
Jesus knows that when his command is genuinely carried out, a friction with the world will almost always arise. For what Jesus proposes counters what the world puts forward. Jesus commands selflessness while the world proposes individualism and self-centeredness. When conflict with the world does take place, Jesus wants his followers to take heart in the fact that they acted in the same way towards him, and that it is Jesus’ words that the world will be rejecting, not them.