Why does God send love into our hearts? Why must we tend this love so diligently?

Why love for God above all else?

Because, "where your treasure is," Jesus said, "there your heart will be also." Our loves move us. If we love wrongly we move wrongly. If crude loves attach us to unholy things, we become crude and unholy. "The foot of the soul is love," wrote St. Augustine, "for it moves by means of love to the place it is going."

The place we long to go is God. The only thing that'll carry us there is love.