On of the most interesting theories about human evolution posits that we learnt to deceive ourselves in order to deceive others. From Wikipedia:
It has been theorized that humans are susceptible to self-deception because most people have emotional attachments to beliefs, which in some cases may be irrational. [...] One deceives oneself to trust something that is not true as to better convince others of that truth. When a person convinces himself of this untrue thing, they better mask the signs of deception.