I think it relates to our beliefs or rationalizations about lying. We see "little white lies" as OK lies because they are the better of two evils, one being telling a lie, and the other being upsetting or hurting someone or getting into trouble.
We all lie sometimes, perhaps without even realizing it. E.g., if a friend asks you "So how do you like my boyfriend?" are you going to say, "I think he's a complete jerk. I can't stand him." I don't think so. Most people might lie and say something innocuous like, "He seems like an interesting guy." And to me, that seems better than alienating a friend.
There are other reasons for little lies too, I suppose, like trying to make a "good impression," saving money on your taxes, trying to fit in with the crowd, etc.