Dhp I: Yamakavagga

"Phenomena are preceded by the heart, ruled by the heart, made of the heart. If you speak or act with a corrupted heart, then suffering follows you — as the wheel of the cart, the track of the ox that pulls it. Phenomena are preceded by the heart, ruled by the heart, made of the heart. If you speak or act with a calm, bright heart, then happiness follows you, like a shadow that never leaves."

I would say all starts in the heart. An action, an intention, a thought all starts from the heart. The untrained does not know the heart, does not observed the heart, and speaks whatever that comes to mind. Just like a novice rider that rampage across the street on a wild elephant.