A liar supposes that if he does not perform his different duties, can justify his violation through false excuses. For such person, not being faithful to promise and covenant, not observing time, not performing assigned duties and such like is easy, because he glosses over the issue with a series of false pretexts and saves himself from any rebuke.
But in contrary, one who is bound to observing the principle of truthfulness and honesty, is obliged to observe such affairs and so can not be improvident and careless.