First John 4:8 says, Whoever does not love does not know God, for God is love.
Of course, everyone loves to some extent or other. More or less all parents love their children, people in sexual maturity generally fall in love at least once, kids love their pets, and so on. Insofar as love is from God (1 Jn 4:7), we might say that these sentiments of love are really from God. They are gift of God which, if we pursue them and try to cultivate them, will lead us to communion with God in a genuine sense. But John here is talking about love in a deeper, more profound sense.
It seems to me that John is referring to love as a fundamental way of being, as the basic orientation we have towards the world.
Think about the most basic way you choose to relate to the world, to other people, and to God. If you think of the world as governed by the law of the jungle, where only the strong will survive, and your primary concern is basically to look out for number one, then your basic orientation towards life might be self-preservation. Alternatively it might be suspicion: perhaps you are fundamentally unwilling to trust people, either out of fear of being hurt, or else because you are suspicious that people are trying to get something out of you.
John says that a person who knows God and who has been born of God will have love as his fundamental orientation towards the world. That person is first and foremost open to the world in love: in a genuine and committed concern for the good of others, even to the point of self-sacrifice when the cause is worthy.
Now, how can you live in this way, when the world is such an unfriendly and hostile environment? Notice what kind of a knowledge is implicit in this attitude! People who are fundamentally suspicious of others, or concerned for self-preservation above everything else, have a certain way of thinking about the world and their place within it. They probably think that death is the end of life tout court, that consciousness ceases and there is no hope of return from the abyss of nothingness. In that light, they are suspicious of any notion of cosmic justice, any hope that things are right in the world and that there is a moral order. Rather, they are deep down convinced that they have to look out for themselves above everything else, and everyone is in a fight to the death for the life-preserving use of limited resources. It is impossible to open yourself up in the way that John demands in a world like this! It's positively irrational!
That is why it is necessary to know God in order to love. First, you must know that God exists -- that he is really there, behind all of our experiences and everything that happens to us and to the world at large. Next, you have to know what God is like. He is made known through his Son, Jesus Christ. And what does Jesus Christ teach us about the Father? That he is love! As John says, Christ is the atoning sacrifice for our sins, and not for our sins only but also for the sins of the whole world (1 Jn 2:2). Not Christ's death in isolation from his life, but Christ himself is the atonement for our sins -- as if the very definition of Christ, the very essence of his identity, the job description on his business card, is to make atonement for sinners and to advocate on their behalf to God the Father. And this shows us what God is like! He is fundamentally, essentially, and immutably for us, pro-humanity, in our favor, a philanthropos.
When you come to realize that God, who is Love, is behind everything that happens to you in the world, that nothing happens outside of the permission of his will and his providence, then and only then can you live in love. At that point, there's nothing to fear! Who or what could ever separate you from the Love of God? What could stand in his way, when he keeps everything in existence from every passing moment?
Now when I think about myself and when I notice the stream of thoughts that goes through my head on a daily basis, I realize how far away I am from this. Love is not yet my fundamental and primary orientation towards life, towards others, towards God. I am yet suspicious, I am yet insecure, I am yet afraid, I am yet resentful, I am yet angry, I fall short in a number of ways. But I want love to be my fundamental orientation; and I think all Christians who have come to see what God is truly like can agree that they at least want to love -- the sign that God has really entered their lives.
The little love that I do feel, I have to cultivate and work at. I have to devote my energies and efforts to strengthening this muscle, rather than the muscles of anger or resentment or self-preservation or whatever else might stand in the way. And to help me, I have to come to know God deeper and deeper: I have to reflect and meditate on the passion of Christ, which teaches me (as T.F. Torrance once said) that God loves us more than he loves himself. Like John of Damascus says, obedience to Christ's commands is only possible with prayer and patience.