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Theologically speaking, through the Holy Spirit we receive the theological virtue of Charity (more commonly called Love), "by which we love God above all things for his own sake, and our neighbor as ourselves for the love of God" (Catechism of the Catholic Church, point 1822).
Point 733 of the Catechism of the Catholic Church states:
"God is Love" and love is his first gift, containing all others. "God's love has been poured into our hearts through the Holy Spirit who has been given to us."
Thus, those who are greatly infused with this virtue will by its sole merits express such love through internal and external acts (minding the issue of sin, temptation, selfishness and other obstacles on the way). Again, God is Love, so receiving God through the Holy Spirit (recall the issue of the Trinity) by definition predisposes our will and soul to love! The reward is a non-issue.
Yet it is true that charity is essential for salvation. Point 837 of the Catechism states:
... Even though incorporated into the Church, one who does not however persevere in charity is not saved. He remains indeed in the bosom of the Church, but 'in body' not 'in heart.'
Saint Paul reminds us (1 Cor 13:1-3):
If I speak in the tongues of men and of angels, but have not love, I am a noisy gong or a clanging cymbal. And if I have prophetic powers, and understand all mysteries and all knowledge, and if I have all faith, so as to remove mountains, but have not love, I am nothing. If I give away all I have, and if I deliver my body to be burned,[a] but have not love, I gain nothing.
Thus, it would have been a bit unfair for Jesus not to let us know that indeed love lead us to heaven. Thus, Jesus is in this sense acknowledging a reality - the means of salvation. He is not suggesting we should love because we would otherwise go to hell. The sole fact that we are made for love is sufficient reason for us to seek the source of this love and make it the aim of our lives.