The typical buyer's decision is usually heavily influenced by the actions of his acquaintances, neighbours or relatives. Thus, if everybody around is investing in a particular stock, the tendency for potential investors is to do the same. But this strategy is bound to backfire in the long run.
No need to say that you should always avoid having the herd mentality if you don't want to lose your hard-earned money in stock markets. The world's greatest investor Warren Buffet...