I'd say most people just say whatever the other wishes to hear, because it's the easiest choice and it has positive consequences on the relationship - at least on the short term. But what others want to hear is not necessarily the truth, far from it.
This presumes that people know what other people want to hear. I find mind reading difficult, if not impossible.
I suppose, in a situation where I knew with good confidence what the other person wants to hear, that I may lie to tell them what they want to hear, but lies have many consequences other than just the immediate "positive consequences", and the long term consequences could be very negative.
For instance, a common lie is to answer "Yes, with pleasure" and smile when asked to do something despite not actually wanting to do it at all. The truth would be more like "I don't want to do that, but I'll do it for you because I like you" or just "No".
I'm not sure how common this is. I can't remember the last time I heard the words "with pleasure" used in response to someone being asked to do something. "Sure. I guess," seems quite a bit more common than either of the answers you give.
But we are all relying on our own ideas of what is common. It would be interesting if anyone measures this sort of thing.
So the "reason" for most lies is simply this - avoiding conflict. From this conclusion I could even say Feeling types are more prone to lie because they often hate conflict and they need to keep a positive self-image of themselves; this includes this very thread and all those who claim they never lie. No offense intended, I'm just looking for the truth.
I suppose A reason to lie would be to avoid conflict. But I can see many possible reasons, including getting something you desire, and various other things.
I have now become interested a bit on studying lying.
Because to me, saying something genuine is simple, you just relay what you genuinely think. As much as this genuineness reflects the truth, you have told the truth. But to lie (which I believe is more than just stating something that is false), you usually have to make something up that is not a reflection of what is in your head.
I wonder if people were "implanted" with an idea that is clearly false, and then asked to give their genuine thought about a situation, if lying would all of a sudden be more common. Because, if that were so, it would lend credence to the idea that "all-else-being-equal" telling the truth is easier because nothing needs to be made up.
Actually, I very vaguely remember hearing about this "folk-experiment" where people were all asked to say something to appease someone regarding something, and that most people just did it. (Kind of an "Emperor's New Clothes" thing)