Honesty entails telling the truth at all times, hence honesty, they say, is the best policy. Any reason for telling lies in committed relationships? Sharing is caring!!!
Lying contains certain mental properties that I believe are worth noting. A person may not have the emotional and cognitive skills to tolerate difficult affect states, e.g., primary emotions like jealousy. A person may have grown up with caregivers/legal guardians who avoided conflicts by not telling the truth to family members. Lastly, a person may identify a value that says that lying is appropriate in certain contexts. Transparency may not always benefit this person, from his viewpoint.
Sometimes people just lie about relatively trivial matters to avoid tiresome discussion, to not hurt someone's feelings, or to save face. This happens both within and outside of committed relationships.