Why is my father being so secretive about his doctor?

My father told us over 12 years ago he was dieing from cancer. During this time he has gone through several different types of treatments (at least he says so) it has now reached the point of diminished returns where he isn’t getting any more treatments and tells us he is going to die, very soon.
He will not let anyone talk to his doctor. Not even know what her name is.
We know that he is taking methadone, and has been for awhile. At least a couple of months now. He looks terrible. There is no doubt that his time is short, I just can’t for the life of me figure out why he is so stuck on this secrecy thing.