We as Christians, want to grow in love, and be able to love not only our friends, and family, but to love even our enemies, as we are commanded.
But if we look carefully at this verse in 1 Thessalonians 3:12, we can see plainly that it is God who makes us love other people, and it is not just us loving them. The Bible says the love of God, is in our hearts. His love in us enables us to love others, and not just our own effort.
In fact this verse says that God can make our love for other to increase, and overflow. That is growing in love. That is how we grow in love. We have to grow in faith, in his grace, in love, and many other areas of our Christian life. And it is the Holy Spirit in us, who makes us grow in love, faith or any other area. It is not by our own decision or will only! We can not make ourselves grow in the love for others. It is God in us, who does that.