The answer is, "YES," there is a scientific reason. It has to do with the neurons, actual nerves from the brain, which are in the heart.
Although this was discovered in the 1970s and early 80s, no one paid much attention to it. Then a company called HeartMath began investigating this heart-brain connection.
They found that the heart could calm the brain down. You could see it on brain wave graphs. The brain waves would synchronize with the heart.
When you focus on your heart you are calming that monkey mind of yours down and changing your body chemistry.