technically

technically, it"s the brain. if you want to classify love as some sort of scientific reaction, then the answer to your question would be that we love from the brain.
the way that we react to things that we love is the reaction that the chemicals in your brain make you react. whether the object of said love is that of a person, place, thing, idea, etc.

but from someone that was to consider love to be something beyond just an anatomical reaction from an organ in our body, they could consider it to be from the heart. scientifically speaking, the heart has absolutely no effect on our emotions, or love. its sole function is as a battery, if you will, that pumps and transports blood throughout our bodies and provides us with the ability to move and live. but the reason that the heart is so commonly associated with love is just as a metaphor. it really stemmed from early beliefs of the ancient Egyptians and Greek philosophers such as Aristotle and Socrates, that human emotions were created by different organs of the human body. they linked the emotions they felt to their souls, and the act of living and possessing a soul itself was credited to the organs. the heart just roughly translated over time as the particular organ that manifested love. possibly due to the effects that love had on the heart, like when it felt broken when sad, or beating fast when in love.