Masturbation becomes a substitute for intercourse in marriage. Sex is designed to communicate love and intimacy with your spouse. Stimulating yourself doesn't. In fact, it's really not sexual stimulation at all, but simply self-stimulation. Licensed marriage counselors are trained to deal with this issue.
Masturbation becomes an uncontrollable habit.
If you find yourself masturbating in public places, it's definitely time for some professional help. For people of faith, there are additional questions. Unfortunately, the Bible says absolutely nothing about self-stimulation. Possibly because it was not an issue in ancient times for reasons we've already discussed. Possibly because God really doesn't care whether you occasionally masturbate or not.