I read somewhere a while back that it's supposedly an evolutionary thing. In a social competition for resource allocation, confidently arguing your position regardless of its correctness is more beneficial than admitting you may be wrong.
It's probably exacerbated by the internet, where the relative anonymity and psychological disconnection further reduces any benefits to admitting to an error.
Interesting. I was thinking more of gray area stuff than outright lying, like playing up the importance of facts that support one's position and downplaying those that don't.