Christian love toward another Christian does not depend upon personal affection or the bonds of common friendship. Christian love grows ultimately from adoption by Father in the Son:
By brotherly love we come nearest to the spirit of the Father. The feelings which are classed under the term “love” vary considerably. Love may be due to admiration for the personal qualities of another, to a common interest in Church matters, to a sense of obligation, the fruit of gratitude; but there is nothing essentially Christian in all that. Brotherly love is to love another because he is our brother, and for no other reason; not because there is anything lovely in him, but just because we have a common father. Brotherly love towards God’s children—that is Divine; that is to be of one spirit with the Father; that is to feel in measure as he does.
Pulpit Commentary on 1 Peter 3:8