There are a few components that play a part in the recurrence of mental illness.
Coexisting stresses in life: often times, a mental illness will recur if there are additional life stressors (family or marital problems, starting a new job, losing someone close to you).
Age: more often than not, if someone is diagnosed with a mental illness earlier than later in life, there is a higher likelihood of the illness recurring.
Diagnosis: Some mental illnesses are more common to recur than others. Some of the ones on the list that are more likely to come back are: OCD, drug and alcohol addiction, eating disorders, and bipolar disorder.