In ancient times people believed that the Heart was the Center of all Human Emotions.
They also believed that the brain served no actual purpose and, in some cases, they are correct.
We know the heart is in the center of the chest and these dummies thought that the heart was therefore the center of emotions. Since love is a strong emotion, the Heart became the Symbol of Love.
Shakespeare said, "Hear my soul speak. Of the very instant that I saw you, Did my heart fly at your service," and "A heart to love, and in that heart, Courage, to make love known."