We think we can look into the eyes of someone and know when they are lying, or when they are being truthful.
Indeed, some of us can for some people. The problem is that none of us really knows with certainty who we can "read", and who we can't. And both game theory and evolution suggest that it will always be neck-and-neck between the ability to tell lies that can't be detected and the ability to detect those lies.
There are advantages to any group of people for all to be (reasonably) truthful. But there can be even greater advantages to an individual if he or she can lie and get away with it. There lies the ability to be a free rider on the efforts of others.
As some people are better at spotting liars than others, so some are better at telling lies than others. Some can be very good at it indeed.
At a TED presentation Pamela Meyer gives some tips on How to spot a liar.