The short answer is that the natural man cannot love properly, and they hate the Lord and His Church (which is the focus here, cf. "let us love one another"), only the regenerate by the work of God can.
For the longer answer I'll give you John Gill's commentary, both on this and 1 John 3:14 which addresses the same issue.
Beloved, let its love one another
The apostle having finished what he proposed to say concerning the trying of spirits, returns to his former exhortation to brotherly love, and which comes with fresh force and strength; for since worldly men follow, hear, embrace, and cleave to the false teachers; such as are of God, and on the side of truth, should love one another, and their faithful ministers, and stand fast in one spirit by the truths of the Gospel, in opposition to every error:
for love is of God:
to love one another is the command of God, it is his revealed will, and is well pleasing in his sight; it comes from him, is a gift of his grace, and a fruit of his Spirit, and which he teaches regenerate ones to exercise:
and everyone that loveth
God, as the Alexandrian copy reads, or Christ, and the saints, who seem to be particularly meant:
is born of God;
for love to the brethren is an evidence of regeneration;
and knoweth God;
he knows God in Christ, and therefore loves those who have the grace of God in them, and the image of Christ upon them; he knows the mind and will of God, being taught of God to love the brethren; and he knows the love of God, and has had an experience of the grace of God, which influences him to love the saints.
We know that we have passed from death to life
From a death in sin, a moral or spiritual death; which lies in a separation from God, Father, Son, and Spirit; in an alienation from the life of God; in a loss of the image of God, of righteousness, holiness, and knowledge, in which man was created; in a privation of all true sense of sin, and in a servitude to it, which is unto death, and is no other than death: and from a legal death, or death in a legal sense, under the sentence of which all men are, as considered in Adam; and which God's elect are sensible of, when convinced by the Spirit of God, and are in their own apprehension as dead men. Now in regeneration, which is a quickening of sinners dead in sin, a resurrection of them from the dead, the people of God pass from this death of sin, and the law, to a life of sanctification, having principles of grace and life implanted in them; and to a life of justification, and of faith on Christ, as the Lord their righteousness; and to a life of communion with Christ; and to such a life as is to the glory of Christ; and to a right to eternal life. And this passing from the one to the other is not of themselves, it is not their own act; no man can quicken himself, or raise himself from the dead; in this men are passive: and so the words are rendered in the Vulgate Latin, Syriac, and Arabic versions, "we know that we are translated"; that is, by God the Father, who delivers from the power of darkness, and death, and translates into the kingdom of his dear Son, which is a state of light and life; or by Christ, who is the resurrection and the life, who is the author of the resurrection from the death of sin to a life of grace; or by the Spirit of life from Christ, by whom souls are quickened, and of whom they are born again: and this passage from death to life, or regeneration, is a thing that may be, and is known by the regenerate man; who, as he knows surely, that whereas he was blind he now sees, so that whereas he was dead in sin, he is now alive; and among other things it may be known by this,
because we love the brethren:
this is not the cause of passing from death to life, but the effect of it, and so an evidence of it, or that by which it is known; brotherly love being what the saints are taught of God in regeneration, and is a fruit of the Spirit of God, and is what true faith works by, and is what shows itself as soon as anything in a regenerate man; nor can anyone love the saints, as such, as brethren in Christ, unless he is born again; a man may indeed love a saint, as a natural relative, as a good neighbour, and because he has done him some good offices, and because of some excellent qualities in him, as a man of learning, sense, candour, civility though he has not the grace of God; but to love him as a child of God, a member of Christ, and because he has his image stamped on him, no man can do this, unless he has received the grace of God; so that this is a certain evidence of it:
he that loveth not [his] brother, abideth in death;
in the death of sin, in a state of nature and unregeneracy; under the sentence of condemnation and death; and he is liable to eternal death, which is the wages of sin, under the power of which such a manifestly is. This is said to deter from hatred, as also what follows.