Why Heart is the symbol/sign of love!

Everybody knows what's the symbol of love, heart & sometimes an arrow pierced through it.

People are quite used to make  statements linking heart like 'use your mind instead of your heart while making a tough decision'. Though its meaning is clear -- choose the right path which may be tough to follow, but it sounds like ancient zen. So & so has a big heart represents generosity of the third person. 
But why heart, it could be anything. What about other parts of the body - kidney, lungs, belly etc.  They are not less important than heart w.r.t. the human body. This thought clicked in my mind some years ago. I did a Google search -"Why heart is the love symbol" & found 13 million sites so I'm not the first who had this thought.