Morphine sulfate is a narcotic analgesic. All people that take narcotics for any extended period of time can develop a physical or psychological dependence to them (or both). Keep in mind that not all people develop either. If possible, it is best to slowly taper the dose off instead of stopping the medication abruptly. Doing this will help the person physically and psychologically adapt to the forthcoming stoppage.
As far as chemical changes to the brain are concerned, per say there are none, however the person may crave the medication, especially if they were on it for a long time. These cravings can definitely lead to mood changes, they may become irritable or anxious. If they believe that nothing else can help with their pain except morphine, this in itself may lead to anxiety.