Dr. Helen figures it is usually mere self-preservation:
I think that often men lie because they will get a very severe response from women if they tell the truth. For example, if a woman says, “What’s wrong?” and rather than reply, “I’m fine,” the man says, “You are driving me crazy and I need some time away from you,” there is a good chance the woman will make him pay dearly for the remark. I don’t know about you, rather than lies, I think many of these quips are more like self-preservation.
Based on my observations, this is a correct conclusion. Men not only learn to lie readily about their thoughts and feelings, they’re so accustomed to seeing the truth provoke insane rage from women that most of them have developed an instinctual reaction to back up the lies of other men even when they have no idea what is going on.
It’s pretty simple rule that actually applies to both sexes. If you genuinely want the truth, then you can’t react with fury whenever someone gives it to you. And once you’ve taught someone that you really don’t want to hear the truth, you’re not likely to hear it very often. I don’t think there’s anything inherently wrong in not wanting to know the truth at all times, so if you suspect that you don’t want to hear it, just leave the matter alone.