I think he basically is doing it to get out of hassle, for example if I ask him whether he has finished packing his school bag, he just says 'yes', if I ask him if he has cleaned up his bedroom he says 'yes' etc etc. All normal teenage stuff. But then it goes on and on, lying about anything and everything, however routine - he lies about what he does in lessons, he lies about where he has put things of ours he has borrowed, he lies about where his own things are, he lies about money, he lies about minor household stuff, and this pretty much happens every single time he opens his mouth.
It came to a head recently when he completely fabricated his mock exam results and accepted praise when they were about two grades out from the official school ones (because he had done zero revision, and yes, he had lied about that too, on a daily basis). Then he lied elaborately over the last two weeks about attending Taekwondo with his friend, which he is supposed to be doing as part of his D of E bronze, something he volunteered for - basically he has just been cycling down to his late's house and hanging out there. He pretended he had his Taekwondo clothes on under his jeans and jumper (does he think I am stupid?) He even came back five minutes before the end of the putative Taekwondo session, let alone the 15 minutes travelling time (yes, he obviously thinks I am totally stupid). He lies about what he can and can't remember, he pretends he can't tell the time, he spends money on DH's credit card and lies that he didn't know what he was doing, he lies that he has met homework deadlines and school responsibilities. There is nothing he doesn't lie about, big or small.
The thing is, I am not a shouty parent or even a particularly demanding one, and I only chase him when absolutely necessary. And nearly all of the lies are so obvious I just say to him, "You have lied about that, haven't you?" and he says "Yes". I refuse to enable this, so I call his bluff a lot.
I have really called his bluff this time, and dobbed him to the D of E organiser, so he won't get his award now. Apart from anything else, he makes me look a right idiot half the time and I really had had enough.
But I still don't get why you would lie so much if there is so little in it for you.