The usual explanations that are given, particulary if you are sitting very close to the person, are either that you might be feeling shy or awkward, or that you might be attempting to deceive the person you are talking to.
Can there be any other reason?
A study indicates another possible explanation at least for some of the times.
We get a great deal of information by looking at faces, and this information places a significant load on our cognitive systems. Perhaps, when we’re asked a difficult question and need to concentrate, looking away from a face helps us focus on the cognitive demands of the question.The next time your better-half chides you for not looking into their eyes during a particularly inconvenient conversation, cite this study and stoutly maintain that you were not lying but were focusing on the cognitive demands that communication necessitates.
It just might help you worm out of a sticky situation.