With Jesus having displayed upon the Cross the ultimate love of God for sinners (Rom 5:8; Eph 2:4-5; 1Jn 3:1) herein we find the exclusive basis for our loving one another (1Jn 4:7,9-11). Neither guilt, debt, reward or mere gratitude, but grace alone makes us to reflect back unto God and toward our neighbor (enemies too) a love He sheds abroad in our hearts by means of the Gospel. In other words, as we behold with faith this love of God revealed in the Gospel text, we too are made to love sinners such as we ourselves are, and this, with a genuinely Spirit-wrought love and gratitude which issues forth from us toward God (1Jn 4:10).
The Gospel alone (that is, the historic, redemptive, and personal work of Christ, 2000 yrs. ago) is the exclusive cause of a God-generated love in us toward others.