When you talk to your boyfriend or girlfriend (or spouse) they seem absent and don't register the conversation. They also avoid eye contact. It is needless to say if someone avoids eye contact it means that they are not interested in you or they are hiding something and are afraid that you will find out through looking them in the eyes.
It is universally known that the eyes are the gates to the soul. If they don't have any eye contact with you it shows that they are hiding something from you. Liars often do that because they know that what they do is dishonest and it is wrong but they still do it. You might choose to confront them on that but the conversation might be futile and cause an argument.