Gustave Le Bon, in his 1895 classic, The Crowd, spoke to why I pay close attention to the prevailing investor sentiment:
Crowds are only cognisant of simple and extreme sentiments; the opinions, ideas, and beliefs suggested to them are accepted or rejected as a whole, and considered as absolute truths or as not less absolute errors. This is always the case with beliefs induced by a process of suggestion instead of engendered by reasoning.