Hachamim say that Saraat was the punishment for those who had spoken spitefully of their fellow man. For instance, it was with Saraat that Miriam was stricken after she about Moshe. She was forced to leave the camp of Yisrael for seven days as she healed from her affliction.
Lashon Ha'Ra then, is considered a most heinous crime. A disparaging remark can linger on to haunt the victim for years. Someone who makes the stray comment that a prospective job applicant looks "untrustworthy" may cost him not only this particular job, but many other job offers as well. Word spreads very easily, especially if it's a bit of juicy gossip.
When someone sees white spots on his skin (the affliction of Saraat), he must go to the Kohen, who will look at them and determine if they constitute Saraat, as a person does not see his own faults (Negaim Perek Bet, Mishnah Bet). Note that it is the expert, the Kohen, who must do the examination.
Why can't the person himself determine his condition?
The answer is clear. The person is not the best objective judge of his own character. He might look in the mirror and see only perfection, carefully avoiding any blemishes obvious to others. Therefore, he might consider the few white spots to be insignificant while the Kohen can see clearly that they are signs of Saraat. Before someone can properly judge himself, he should solicit an unbiased opinion from others.
Hopefully, we will learn to always humble ourselves, watch what we say and listen when others offer us advice about our own behavior.