心 is a picture of a physical heart — the curved stroke is the outline of the organ, and the three dots are the chambers and the aorta. The ancient Chinese, like many cultures, located thought and feeling in the heart, not the head.
In Chinese, the heart is where you both feel AND think — 心 means heart and mind at once. It fills emotional words everywhere: 开心 (happy — 'open heart'), 小心 (careful — 'small heart'), 担心 (worried), 爱心 (loving-kindness). When 心 shrinks to the side of a character it becomes 忄, as in 忙 (busy) and 快 (fast/happy).
The curved outline of a HEART with three dots for its chambers.
小心 ('small heart') means 'be careful' — keep your heart small and watchful. 开心 ('open heart') means happy. The state of your 心 is the state of you.