People become primitive and action-oriented when they join crowds. Crowds feel simple but strong emotions such as terror, elation, alarm, and joy. Crowds swing from fear to glee, from panic to mirth. A scientist can be cool an rational in his lab but make harebrained trades after being swept up in the mass hysteria of the market. A group can suck you in, whether you trade from a crowded brokerage office or from a remote mountaintop. When you let others influence your trading decisions, you lose your chance of success.
– Alexander Elder, Trading for a Living