When affection for an individual or a thing reaches the summit of intensity so that it conquers man's existence and becomes the absolute ruler over his being, it is called love. Love is the peak of affection and the sentiments.
But it should not be imagined that what is called by this name is of only one kind; it is of two completely opposite kinds. Those things which are called its good effects are connected with one of its kinds, but its other kind has completely destructive and opposite effects.
The sentiments of man are of various kinds and degrees; some of them are in the category of the passions, especially the sexual passions, and are of those aspects which are shared by man and the other animals, with the difference that in man, for a particular reason the explanation of which cannot be appropriately undertaken now, it reaches its peak and takes on an indescribable intensity; and for this reason it is called love. It never takes on this form among animals, but, in any case, in its reality and essence, it is nothing but a torrent, a bursting forth, a tempest of the passions. It originates from the source of sexuality, and reaches its end there too. Its rise and fall are, too a large degree, connected to the physiological activity of the genital organs and naturally to the years of youth; it diminishes and eventually ceases altogether with an increase in age, on the one hand, and, on the other, with satiation and separation.
A youth who feels himself a-quiver at the sight of a beautiful face or a tress of hair, or who feels a frisson when touched by a tender hand, should know that there is nothing more operating here than a material, animal process. This kind of love comes quickly and goes quickly. It cannot be depended on, nor recommended, it has dangers and it kills virtue. It is only by the help of modesty and piety and not becoming abandoned to it that it may profit a human being; that is, in itself, it is a power which leads man towards no virtue. But it gives a strength and a perfection to the spirit, if it penetrates into a man's being, is met with the power of modesty and piety, and if the spirit tolerates the pressure of it - provided it does not succumb to it.
Humans have another variety of sentiments which, in their reality and essence, differ from the passions; it is better to call these noble sentiments, or in the language of the Qur'an, "love and mercy" (muwaddah wa rahmah [see 30:21 ] ).
As long as man is under the control of his passions, he has not gone out from his self, he seeks a person or a thing whom he is attracted to for himself, and he wants it dearly. If he thinks about a love-object, it is with the idea of how he might profit from being united with it, or at the most how he can derive enjoyment from it. It is obvious that such a state cannot be the perfecter or the educator of man's spirit, or refine it.
However, man occasionally comes under the effect of his higher human sentiments; his loved-one receives respect and eminence in his eyes, he seeks that person's happiness. He is prepared to sacrifice himself for that person's desires. This kind of sentiment brings purity, sincerity, tenderness, compassion and altruism into existence, as opposed to the first kind which creates crudeness, savagery and criminality. The kindness and affection of a mother for her child is of this second kind. Devotion to, and love of, the pure ones and the men of God, as also patriotism and the love of principles, are also from the same category.
It is this kind of sentiment from which, if it reaches its summit and perfection, all the aforementioned good effects result; and it is this kind which gives dignity, distinction and greatness to the spirit, in contrast to the first kind which brings wretchedness. Similarly it is this kind of love which is durable, and which becomes stronger and warmer with union, as opposed to the first kind which is not permanent and whose graveyard union is reckoned to be.
In the Qur'an, the relationship between a man and wife is described as "love and compassion"  , and this is a very great point. It is an indication of the human and higher than-animal aspect of married life. It is an indication that the factors of the passions are not the only natural link in married life. The fundamental link is purity, sincerity and the union of two spirits; or, in other words, the thing which joins the married couple one to the other, and unites them, is compassion, mercy, purity and sincerity, not the passions, which also exist between animals.
In his own subtle way, Rumi distinguishes between the passions and true love; he calls the former animal and the latter human. He says:
Wrath and passion are the attributes of beasts,
Love and compassion the attributes of man.
Thus Love is the characteristic of
Adam, missing in animals, a deficiency.
Materialist philosophers too have not been able to deny this spiritual state which, from several standpoints, has a non-material aspect, and which would not be in conformity with man and what is beyond him being material.
In Marriage and Morals, Bertrand Russell writes:
Work of which the motive is solely pecuniary cannot have this value, but only work which embodies some kind of devotion, whether to persons, to things, or merely to a vision. And love itself is worthless when it is merely possessive; it is then on a level with work which is purely pecuniary. In order to have the kind of value which we are speaking, love must feel the ego of the beloved person as important as one's own ego, and must realise the other's feelings and wishes as though they were one's own. 
Another point which should be mentioned and carefully attended to is that we said that even loves of the passions may possibly become beneficial, and that occurs when they become linked to piety and modesty. That is to say, in connection with, on the one hand, separation and inaccessibility, and, on the other hand, purity and modesty, the pains and anguishes, pressures and difficulties to which the spirit is subjected bear good and beneficial results.
It is in this connection that the mystics say that allegorical love is turned in real love, i.e., love of the Essence of the One; and it is also in connection with this that the following tradition is narrated:
He who becomes a lover, who conceals (his love), who is chaste (in his love) and dies (in that state) has died as a martyr.
However the point must not be forgotten that this kind of love, with all the advantages that may, under particular conditions, possibly be brought about, is not to be recommended - it is a dangerous valley to enter. It is in this respect like an affliction, which, if it troubles someone and he opposes it with the force of his patience and will, becomes a perfecter and purifier of his soul; it cooks what is raw in it and clarifies what is turbid in it. But one cannot recommend an affliction. No-one can create an affliction for himself so as to profit from these factors which prepare and train the soul; neither should he bring about an affliction for someone else on this pretext.
Here, also, Russell has something valuable to say:
Suffering fills people with energy, like an invaluable counterweight. Someone who deems himself to be entirely contented will not exert himself any further for happiness. But I do not advocate that this be made a pretext for causing others suffering so that they may tread a profitable path, because it often gives the opposite result and destroys man. Rather, it is better in this case to submit our own selves to chance events that fall in our way. 
As far as we know, the effects and advantages of afflictions and misfortunes have been much emphasized in Islamic teachings, and they are well-known as signs of God, but this in no way permits anyone to create afflictions for himself or for others on this pretext.
Moreover, there is a difference between love an affliction; and that is that love, more than any other factor, is against reason. Wherever it sets foot, it ousts reason from its governing position. This is why love and reason are well-known in mystic literature as two rivals. The antagonism between the philosophers and the mystics originates from here, the former depending on, and confiding in, the power of reason, the latter in the power of love. In Sufi literature, reason is always condemned and defeated in this field of competition. Sa'di says:
My well-wishers advise me
It is useless to make bricks on the sea.
But the power of yearning prevails over patience:
The pretension of the intellect over love is futile.
Another poet has said:
I drew a comparison for the counsel of reason in the path of love:
It is like a fall of dew trying to trace a pattern on the sea.
How can a force which is as powerful as this, which snatches the reins of the will out of our hands, and which, in the words of Rumi "blows a man here and there like a blade of straw in the hands of a fierce wind", and in the words of Russell "is something with propensity for anarchy", be recommendable?
At any rate, it is one thing to happen to have useful results, but it is another to be advisable or recommendable.
From this it will be seen that the objection and complaint which some Islamic jurists have levelled against some of the Islamic philosophers' who have set forth this matter in their metaphysics and have explained its results and advantages, is invalid. For the former imagined that the opinion of the latter group of philosophers was that this matter is both advisable and recommendable, whereas they only considered the useful effects of this kind of love which appear under conditions of piety and chastity, without recommending or advising it, just as they would have done with afflictions or misfortunes.
And of His signs is that He created for you, of yourselves, spouses, that you might repose in them, and He has set between you love and compassion. (ar-Rum, 30:21 )
. B. Russell: Marriage and Morals, London, 1976, p.86
. ibid. Translated from the Persian, original untraced.
. Ibn Sina in his Treatise on Love (Risal-a `ishq), and Sadru'd Din ash-Shirazi in the third journey of his Asfar al-arba`.