Truthfulness Reflects God's Character.
Lying is deception, and God never deceives. We rely on God's Word that it's trustworthy and true. If that were not the case, we would have no standard for morality in a world where everything is relative. The Scriptures declare that God's Words are true, righteous, eternal, and a light for our path. They are the plumb line for all that is righteous and holy. Therefore we believe in faith that God is truth and we ought to be the people of the truth.

Falsehoods Reflect the Evil One.
In John 8:44, Jesus declares that Satan is the father of lies and a murderer. Interestingly, murdering and lying are related: Murdering steals a life, and lying steals the truth, and both reflect the character of the Evil One. Every time we declare truthful what it is not (lie), we reflect Satan rather than God. This is why deception (which is what a lie is) is wrong. It's impossible for God to lie, but Satan by nature is the father of all lies.

Truth Builds Integrity and Trust.
Integrity is about being whole and undivided. The man of integrity is honest and truthful in all things which means that others can entirely trust his word at all times. People value an honest individual because they can be trusted. It's not surprising, but any reflection of God's character works every time. Lying fuels mistrust, while honesty fuels trust. Nothing destroys trust in a marriage faster than deception. Or in the workplace. Or with a friend. Or in a church. And nothing produces trust more than honesty in all things. Honest people can be trusted while dishonest people cannot.

And besides, those who are truthful with others are also truthful with themselves. Those who lie to others will find it easy to lie to themselves which is a prelude to personal failure. Those who are honest with themselves can learn and grow while those who do not simply gloss over their issues when it is convenient. Thus honesty and integrity apply equally to what we tell others and what we tell ourselves.