Well... I wish I knew.
I'm not perfect by any means and I know that I have lied or been dishonest by omission many times. But in general, I strive for honesty. And I greatly admire honesty in others. And I will have a hard time trusting someone that has deceived me. I value honestly greatly.
I think the truth is always the best. Even when it hurts. If my clothes look terrible on me and make me look like a Weeble, then I want someone to tell me. That's far less hurtful than letting me walk around like that. If someone has wronged me, I would rather them admit it so we can work through things than for them to lie to cover it up.
But sometimes a lie can be so much easier. Or so it seems. I think lies always end up very complicated. But in that moment... everything inside can scream "The only way out is a lie!"
Sometimes we lie to spare somebody's feelings. We lie to save our skin. Save our job. Save our family. I know people who lie for absolutely no reason at all. And they don't even seem to care too much about sounding credible.
And why do Christians lie? Because we aren't perfect. How I wish that the minute I gave my life to Christ that I automatically achieved perfection! How cool would that be?! But no. We still struggle with, and give in to, temptation. We should be giving in less than we did before becoming a Christian. We should be able to see progress in our lives. But perfection isn't going to happen in this body or on this earth.
I have no defense, though, for someone who lies. It is sin. The same is true when I lie. It is wrong. But we are supposed to forgive. Forgiving deceit can be very difficult, but it isn't impossible. And forgiving doesn't mean that we have to give that person our full trust. That is a consequence that must be faced and the person who lied will have to earn back that trust.