The purity of heart required for love is achieved in the first place through union with God in prayer. When God's grace touches us, this also produces a path to pure, undivided human love. A chaste person can love with a sincere and undivided heart.
When we turn to God with a sincere intention, he transforms our hearts. He gives us the strength to correspond to his will. (YOUCAT question 463)
~R.A.
