Some doctors are more conservative and others more aggressive.
Another doctor might come up with a completely different and promising option - one that your first doctor didn't think of, or didn't know about. It happens - no doctor can know everything or make the right decision all the time. A second opinion can also serve a general quality check - to make sure you're really getting the most up to date, most effective treatment. When Here is a list of some reasons why you might want a second opinion. This should help focus your thinking, but there is no cookbook answer for when you need a second opinion.