How does our body produce this gentle warm feeling in our chest, when we experience emotions like happiness, love, compassion, etc?

I read this health article on http://www.msnbc.msn.com/id/11023208/, that explains that the heart can sense a human’s emotional state. In this part of the article it says, “Conversely, HeartMath’s research shows that when we experience heart-felt emotions like love, care, appreciation and compassion, the heart produces a very different rhythm. In this case it is a smooth pattern that looks like gently rolling hills. Harmonious heart rhythms, which reflect positive emotions, are considered to be indicators of cardiovascular efficiency and nervous system balance. This lets the brain know that the heart feels good and often (creates a gentle warm feeling in the area of the heart).” Do you have a scientific explanation for how this works and happens?