Humans are herd animals who seek to be accepted by ‘in-groups’ at whatever level. Plus, we’re designed to be ’lazy’. Both these things make us susceptible to (and easily influenced by) the opinions of others. For some reason, we assume that other people are smarter and more conscientious than we are. One of Robert Cialdini’s seven principles of Influence.
Note: Knowledge of this effect offers no meaningful protection against its effects.