why did people back in the day (i’m thinking…16th century) think that the liver was the source of love in the human body?
To be precise, the liver was associated with physical desire, not love.
It’s difficult to say exactly why the natural philosophers made this association, as they had a complex and often contradictory system of correspondences with colour, astrology, etc.
One of the reasons was apparantly that the bile produced by the liver is green, and green is associated with generative growth and life. Don’t be put off by the smell – the alchemists believed that all growth began with corruption and decay.
(This belief was already well-established by the 14th century, by the way.)
Why do people now associate love with the heart? Associating it with the liver is no less logical.
One reason is that after an animal dies, the liver is the part which stays warm for the longest time. And it’s closer to the center of the body than is the heart.
[QUOTE=Chronos]
Why do people now associate love with the heart? Associating it with the liver is no less logical.QUOTE]
Because your chest hurts when you feel broken-hearted and fluttery when you feel in love? I don’t remember ever having my liver feel any particular way. 
Actually, I feel it more in my solar plexus, or a little above - exactly where the liver is. (It’s a big sucker, lays across most of the abdomen.)