Why do guys lie?

This guy told me he didn't have a girlfriend but said the girl in the picture was his sister. Now he claims she's his girlfriend but still flirts with me. He lied about not drinking and partying. He said he didn't do anything the whole week and I just found out by his friend he was partying the whole week. He told me he didn't drink or smoke either. I see he's drinking and smoking from the party his friends threw on Facebook. We're not even together or talking. At most we flirt and text
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I haven't had to deal with a lying sob for over 10 years. The people around me do not lie bc they know I'll cut them off the minute I find out. My ex spoiled me with the truth no matter how blunt it was. I appreciated that a lot more than going back to dealing with living human shit
Why do guys lie?
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