why people lies

Well, I am a very faithful girlfriend one time but I had a boyfriend who never told me anything but lies. I just don't know why he lied to me much of the time while I was always that sweet, honest girl with him. I hate liars eventhough sometimes I lie to people. My boyfriend lied to me. He didn't tell me that he already had a son. When i found out about it we were already two years in the relationship. I left him. Because I hate liars.