Market sentiment is the collective reflection of emotions like greed, fear, optimism, and pessimism from all market participants. It's like a powerful tide that can overwhelm individual rational judgment for a period of time. Extremes in sentiment signal turning points: when market sentiment reaches extreme optimism, it means almost all potential buyers have entered the market, follow-up funds dry up, and the top is near; Conversely, when the market falls into complete pessimism, those who need to sell have already exited, selling pressure is exhausted, and the bottom is not far away. This is the deeper logic behind 'Be fearful when others are greedy, and greedy when others are fearful.'