A friend, Ute Shaw, asks why we seem too eager to believe the first opinion we hear on a subject, and then seem too skeptical about further contrary opinions we hear. Some possibilities:
We like to appear knowledgeable by having opinions on many topics, and changing our opinion would undermine that appearance.
To show loyalty to our group, we adopt their opinions. Changing opinions would suggested changed loyalties.
To quickly assimilate cultural knowledge, humans are programmed to accept the first opinion they hear. A topic on which contrary opinions are also expressed tends to contain less cultural knowledge.