For many modern people, it’s difficult to understand why you would love from the heart, when we are taught from early childhood that the brain is where our essence seems to reside. As with many common beliefs, a better understanding requires looking far back in history to Ancient Greece. The Ancient Greeks believed that the heart was the seat of everything, in much the same way that most modern people believe that the brain is central to what makes people themselves. Aristotle taught that the heart controlled all reason, emotion, and even day to day thoughts.
From Aristotle, the teaching of the heart as the place of emotions passed on the Romans. The physician Galen, who laid the groundwork for a great deal of later medicine in the West, had an entire theory of the circulatory system. In this theory, the heart was said to be where emotions took place, while rational thought took place in the brain, and passions originated in the liver.
The Egyptians also placed a great deal of importance on the heart as an organ, and many of them likely would also have understood what it was to love from the heart. In Egyptian mythology, the heart was said to contain some vital essence of the soul, such that at death it was weighed against a feather to decide if a person would go on to paradise.
Most modern people have been taught that the brain is where everything about them takes place, so it’s difficult to imagine, in a literal sense, that the heart is connected to love. Most emotions are associated with other organs in common speech, however. People get butterflies in the stomach when they're nervous, feel a sense of something in the gut, and deep pain wrenches the heart from the breasts. This all makes sense, when looking a bit more deeply at the physiological responses to emotion and the fact that people feel them most acutely in organs other than the brain.
Indeed, to understand love from the heart, one need look no further than very basic physiology that most people have likely experienced personally, and which the Greeks themselves likely based their own beliefs on. The state of excitement, a state definitely connected to both love and sexual attraction, is felt first and foremost in the heart. As the body prepares for something exciting, the heart rate increases to increase blood flow throughout the body. This rapid heartbeat, more than any thoughts that might cross through the brain or rumbling in the stomach, is likely what has forever associated that organ as the organ of love.