You do it because talking involves little effort, and you want to share your knowledge. That might be due to a variety of reasons; from trying to prove someone

You do it because talking involves little effort, and you want to share your knowledge. That might be due to a variety of reasons; from trying to prove someone wrong, to a general feeling of wanting to display knowledge. But none of those motivations require that you be, in fact, correct. I've lost track of the times people overheard me discussing something with someone and piped in to tell me the virtues of homeopathy or how GMOs are a plot to destroy the food supply or that there's no actual proof of evolution. I'm most skeptical of people who are least skeptical of their own knowledge and how they acquired it. Yet it seems those are the people most likely to offer an opinion. If you (rightly) skeptical of my claims go find a newspaper and read the letters to the editor. These are people sufficiently convinced of their rightness that they took the time to send a letter or an E-mail.