The ancient Egyptians considered the heart the center of the body. To early Christians, the heart was the seat of the soul. Because love is a centering force, the heart became associated with love and happiness.
Philosophers of old considered the heart the place of thought and intelligence, corresponding to the sun. In heraldry, the heart was used as a symbol of illumination and happiness.
As for we who tat, the heart is a favorite motif to make for ourselves and others. It's usually symmetrical, so after one side is tatted, the second side is a logical mirror. Sewn onto clothing or linens, tucked into a card or letter, the smile it inspires mirrors the up-curving sides of the heart itself.
Some of the hearts inside are easy to tat, others will be a bit of a challenge.