Because you know it’s wrong to lie, doing so "goes against what you deem as 'right,' and builds anxiety," Walfish says. The anxiety just increases as you try to keep from being caught. "A person who lies doesn’t want to be found out. They want the whole thing to go away," she says.
https://www.nextavenue.org/truth-lying-makes-you-sick/
Why Lying Makes You Sick