Is paying someone a complement when we don't really need it a lie? Before we can really answer the question of why people lie, all of these issues is the first addressed in order for us to get a better understanding. Some people are serial liars and agents cannot help of a the able to for these individuals and it is pathological liars. It is usually a cry for attention. They are so obsessed with being seen as interesting, unusual, or perhaps different than they are, that they will say or do anything that might be focus on them. Some of the summit are so incredible that they even realize that no one will believe them, if they do it anyway.
But this is perhaps a simplification of why people lie, so I think we have the dig a little deeper. In many respects figuring out why people is an exercise in common sense. Think about it, a person lies usually to impress others or to get out of a bad situation. Maybe they want to get away with not having done their homework, or having been late for work. Kind of like the classic "the dog ate my homework." Many times people lie because they want to impress others and so they embellish their accomplishments. It is hard to tell in most situations why people lie, that's the tricky part, the easier part is to tell when they are lying.
Body language is one of the best cues to spot a liar. Regardless of why people lie, they usually give themselves away once you know what telltale signs to look for. They don't necessarily fidget a lot, but often they will get stiffer than normal. To contrast this, they might use exaggerated gestures or speak with different language than they usually do. A lack of eye contact is also a pretty clear signal. The bottom line is, that there are many explanations and reasons for why people lie, but with a little education and becomes rather easy to spot when people are lying.
Morgan Hamilton offers his findings and insights regarding why people lie. You can get interesting and informative information here at Why People Lie