Though the apostle does not mention a specific reason, the wider context, with its reference to the work of the Lord in causing them to be born again and so becoming heirs of the kingdom of heaven (= “salvation”), indicates that God’s work of mercy for them is the cause of their love for their Lord and Savior.
8 Though you have not seen him, you love him. Though you do not now see him, you believe in him and rejoice with joy that is inexpressible and filled with glory,