Cause you'll lose trust.
If you're not trustworthy, your knowledge will be hard to be acknowledged by others.
Imagine that, because of a lie, you put others in grave danger without you even realising!
My, that'll be a big problem!
If you've ever lied, do apologise to whom you've lied to.
Repent and ask for Allah's forgiveness for He is all forgiving.
Be truthful cause it's the right thing to do.
And remember that good is not always right but right is always good.
Though sometimes truth hurts, I guess it's more painful to discover the truth hidden behind a lie.