What is Dr. Jekyll hiding?

When Mr. Utterson first brings up his knowing of Mr. Hyde, Dr. Jekyll goes from a cheerful face to one of paleness and worry.  He is obviously distraught about his good friend finding out about his associate Mr. Hyde.  As of now the reason for Dr. Jekyll's dissatisfaction is unknown, but he is holding back a secret that he does not want his friend getting involved in.  What I keep wondering is why Dr. Jekyll is so hesitant about telling Mr. Utterson about Mr. Hyde.  I believe Mr. Hyde is a dangerous person, and that Dr. Jekyll knows this and is just trying to keep his friend safe, but at the same time I feel like Dr. Jekyll is in over his head and might benefit from telling someone his problems and the secret of Mr. Hyde.  I am excited to see what this story will reveal.
Christopher Craig