Here's an excerpt from Judge Alex Kozinski's opinion in US v Xavier Alvarez (PDF), in which the judge describes some of the reasons that people lie:
Saints may always tell the truth, but for mortals living means lying. We lie to protect our privacy ("No, I don't live around here"); to avoid hurt feelings ("Friday is my study night"); to make others feel better ("Gee you've gotten skinny"); to avoid recriminations ("I only lost $10 at poker"); to prevent grief ("The doc says you're getting better"); to maintain domestic tranquility ("She’s just a friend"); to avoid social stigma ("I just haven't met the right woman"); for career advancement ("I'm sooo lucky to have a smart boss like you"); to avoid being lonely ("I love opera"); to eliminate a rival ("He has a boyfriend"); to achieve an objective ("But I love you so much"); to defeat an objective ("I'm allergic to latex"); to make an exit ("It's not you, it's me"); to delay the inevitable ("The check is in the mail"); to communicate displeasure ("There's nothing wrong"); to get someone off your back ("I'll call you about lunch"); to escape a nudnik ("My mother's on the other line"); to namedrop ("We go way back"); to set up a surprise party ("I need help moving the piano"); to buy time ("I'm on my way"); to keep up appearances ("We're not talking divorce"); to avoid taking out the trash ("My back hurts"); to duck an obligation ("I've got a headache"); to maintain a public image ("I go to church every Sunday"); to make a point ("Ich bin ein Berliner"); to save face ("I had too much to drink"); to humor ("Correct as usual, King Friday"); to avoid embarrassment ("That wasn't me"); to curry favor ("I've read all your books"); to get a clerkship ("You're the greatest living jurist"); to save a dollar ("I gave at the office"); or to maintain innocence ("There are eight tiny reindeer on the rooftop")….
An important aspect of personal autonomy is the right to shape one’s public and private persona by choosing when to tell the truth about oneself, when to conceal, and when to deceive. Of course, lies are often disbelieved or discovered, and that, too, is part of the push and pull of social intercourse. But it’s critical to leave such interactions in private hands, so that we can make choices about who we are. How can you develop a reputation as a straight shooter if lying is not an option?