I know this will trigger the usual Hillery tirade but you asked and I'll answer.

If a person is a liar, what good are they in your life? [except for your children--a bond that cannot be severed--and a different conversation]

Let's keep it simple suppose you found out your accountant, lawyer, doctor or wife is a liar. Maybe they lie just some of the time, or just a little--who cares?

Once trust is gone, the relationship is over.