"These things I command you, that you love one another. If the world hates you, you know that it hated Me before it hated you. If you were of the world, the world would love its own. Yet because you are not of the world, but I chose you out of the world, therefore the world hates you. Remember the word that I said to you, 'A servant is not greater than his master.' If they persecuted Me, they will also persecute you. If they kept My word, they will keep yours also. But all these things they will do to you for My name's sake, because they do not know Him who sent Me. If I had not come and spoken to them, they would have no sin, but now they have no excuse for their sin. He who hates Me hates My Father also. If I had not done among them the works which no one else did, they would have no sin, but now they have seen and also hated both Me and My Father. But this happened that the word might be fulfilled which is written in their law, 'They hated Me without a cause.'"
There are many good reasons why Jesus wanted His disciples to genuinely love one another, but in the context, the reason may be that they would especially need the support and kindness of their brethren since the world would hate them.
"If the world hates you, you know that it hated Me before it hated you" (cf. Matt. 10:22ff; I John 3:13). Jesus warns them of this fact in order to prevent them from becoming disillusioned later. Although the world would manifest hatred toward them, they could take comfort in knowing that their Master and Friend had also endured such. They could escape this hatred by becoming friends with the world (i.e., by failing to stand and speak for what is right and against what is wrong). However, being friends with the world creates enmity from a different source, and a much more serious one--God (cf. James 4:4; I John 2:15).
"A servant is not greater than his master" (cf. John 13:16). The specific application here of this truth is that those who persecute the master will certainly persecute the servants. Additionally, those who obey the words of the master will also obey the words of the servants. Essentially, genuine followers of Christ should expect to be treated and received as Jesus Himself was (cf. II Tim. 3:12; I Pet. 4:12,13; Luke 6:26,40).
"If I had not come and spoken to them, they would have no sin but now they have no excuse for their sin" - Jesus is not talking about all of their sins in general, otherwise He would be implying that those who had lived prior to His incarnation would have been sinless. Certainly that is not the case. Thus, Jesus must be referring to the sin of rejecting Him. Had He not come to Earth, taught, and worked many signs (cf. John 15:24), then the Jews would not have had the sin of rejecting Him as a mark against them; they would have had a valid excuse on this matter. However, Jesus did come, teach, and work many great miracles. The Jews had every opportunity to accept Him, and their many opportunities made their sin here all the worse (cf. Matt. 11:20-24; Luke 12:47,48). They were blind, but their blindness was self-imposed! They rejected both Jesus and His Father.
Without the miracles He performed, Jesus would have rightly been labeled a blasphemer in the eyes of all men. However, since He had confirmed His message repeatedly as being of a divine origin, those who rejected Him were without excuse. Jesus' works were of such a nature that the only logical conclusion was that He was from God and that He was who He claimed to be (cf. John 3:2). The reason for their rejection is simple--they hate God!
Their unjustified hatred toward Christ was foretold in Psalm 35:19 - "They hated Me without a cause" (cf. Psa. 109:3-5). Though their greatest act of hatred had not yet come to pass, its hour was drawing near.
Ironically, the Old law (mentioned in John 15:25), which the Jews claimed to love and cherish so deeply, was the document that foretold their rejection of the Messiah! It made their actions all the more inexcusable! Perhaps Jesus is here emphasizing this irony by referring to it as their law instead of the law.