The basis of sympathy is love. Love is the essence, of which sympathy is one of the modifications or forms. It is the nature of pure or holy love, not only to seek the good of others, but, harmonizing with the peculiarities of their situation, to rejoice in their joys, and to grieve in their sorrows. If we truly love others, it will be a necessary result that we shall take an interest in everything which concerns them. Love, taking this form, is sympathy.
We will endeavor to give some illustrations of this interesting state of mind. A truly pious person, one in whom the principle of holy love predominates, is a member of a family. It does not make any difference, in relation to the subject under consideration, whether he is a member by the ties of relationship, or a member by mere residence. One of the members of the family is severely afflicted with sickness. The occurrence of this affliction furnishes the occasion on which the principle of holy love, moved by its own law of action, assumes the form of sympathy. The person who is the resident of the family, being such as we have described him to be, cannot witness such an affliction without "weeping with him who weeps." His sympathy, in the existing state of his mind, is a sort of necessity to him. It is possible that it may not present the same aspect with the sympathy of unsanctified nature, which is often agitated by fear, and perverted by selfishness. But, always necessary and certain in its existence, it will be of that tender, judicious, and permanent character, which will be the most useful, besides being the most heavenly.
We will suppose, again, not that the persons around us are sick, but that they have been deprived of the means of knowledge, and are exceedingly ignorant. They are excluded from science and literature, even in their simplest forms. The Bible, with its precious consolations, is a sealed book to them. It is impossible that they should experience such deprivations without being afflicted; and it is impossible that holy persons, filled with the love of God and man, should be acquainted with their situation, without sympathy. That is to say, under the impulse of love, they suffer with those afflicted ones at the same time that they desire to relieve their sufferings; the term sympathy, expressing, in this case, the combined feeling of sorrow for their want, and of benevolent desire for its alleviation.
— A Treatise on Divine Union (1851) Part 4, Chapter 7.