A sick person goes to a doctor to find out what is wrong with his body. In the same way, every person should find out how to heal his soul. Someone who is not careful with Loshon Hora has a very ill soul - and unfortunately, he often doesn't even realize it.
Imagine if a sick person didn't realize he was sick. He wouldn't go to a doctor, he wouldn't take care of his illness - and he certainly wouldn't get better. A person who doesn't realize he has the disease of Loshon Hora won't try to find ways to cure it.
So what's the solution? Says the Chafetz Chaim: learn the introduction to the "Chafetz Chaim". There, the symptoms are spelled out, so we will realize that we do have an illness of the soul - and that we need Hashem's help to cure it.