Why do people lie?

So I know someone who constantly lies to their boyfriend about stupid insignificant things like whether or not she looked at someones picture or went on their page online and stuff like that. She always says no and the guy knows she is lying. She doesn't wnat to make her boyfriend jealous which is her excuse. She doesn't do anything bad and is a good girfriend other than the fact that she is not honest. Do you think she is not comfortable with herself? Let me know cuz it worries me cuz she is so upset with herself and i want to figure out why she does this so i can help. Thanks!