Medical releases are only required when you answer "Yes" to one of the questions on the medical form. The RSTC (Recreational Scuba Training Council) sets the standards for medicals. They have set the standard at one year to insure the safety of our divers.
Though you may not have had a change in your condition there may be other factors involved. Having you obtain a new medical release for classes after a year gives your physician a chance to review your history and discuss any concerns or suggestions for diving. Medications can change, standards can change, treatments can change, new drugs are always available.
You are only required to get a new release if your medical is over one year old AND you are taking a class. However, why not download a new medical each year and take it on a regular visit and have your doctor complete it agian. It can't hurt and you will have peace of mind.