If a person didn't like his relationship partner but was married for years then he might claim that he loves his partner just because he doesn't want all of the effort he invested to be gone.
A person can lie to himself in order to believe that he has lots of options. A person can claim that he chose something because he liked it and feel good instead of admitting that he couldn't get something better.
A person can create a lie and believe it just to feel good about himself. A person can convince himself that he doesn't want to get rich because money is bad just because he don't want to admit that he hates hard work.
A person can lie to himself to escape from reality or to deny a fact. A person can claim that he hates noise just because he feels uncomfortable in social situations.
A person can lie to himself because he has no enough courage to face a problem. A person might claim that he is unlucky instead of admitting that he didn't try hard enough (See why some people believe in luck).
A person can lie to himself in order to maintain his psychological stability. For example by believing that all rich people are thieves a person can feel good about himself and not feel bad about his lack of money.
A person can lie to himself because he is too lazy to do hard work. A person can blame his weight on his genes instead of tying to exercise in order to save himself the effort.
A person can lie to himself if he believed that the truth might hurt him so much. In such a case the subconscious mind creates a lie and the person believes in it.
A person can lie to himself to allow himself to give up without feeling bad. A person can claim that he didn't really want something that much instead of admitting that he just gave up (See bad moods explained).
A person can lie to himself to keep the good image in front of others. Instead of admitting that he didn't succeed in something a person can claim that he didn't like it.
It's much easier for some people to deceive themselves than to face a harsh reality that they can't deal with.