It's a poem about self-deception, and Frost has cleverly extended that theme to encompass the reader.

He clearly wrote it the way he did to invite an easy, comfortable interpretation, one that is blatantly contradicted by the very evidence he presents. He's rubbing our noses in our own gullibility and tendency to deceive ourselves with comfortable lies, just as the narrator has done.

All in simple, direct language. Frost sure was a sneaky one.