Female labrador retriever very shy

Question
Our dog is very well behaved, but she prefers to retreat to her open kennel almost all the time.  She does come out to greet us if we come in from being away for a time, to eat or to go out, and to come to bed at night.  She either sleeps with us or beside us on a blanket, but even when we call her, if she does happen to come out of her kennel, she comes out with her tail tucked and her head down.  We have never, ever spanked or physically punished her, so we are totally at a loss as to why she behaves this way.  She is 4 years old, and we have had her since she was 3 months old.  Please help!!!

Answer
I have noticed a number of our females sort of became a touch me not towards the end of their first year.  I have even started petting one and had her get up and go in her crate.  Usually as they get older, they do start wanting more contact.  It is possible yours is worse having lacked enough contact with people before she was 12 weeks old.  The 6-12 week period has a great influence on what a dog is like the rest of its life.