Our Sages taught that Lashon Hara (gossip) harms 1.) The gossiper, 2.) The listener and 3.) The one about whom the gossip is spoken (see Arachin 15b).

Now it is easy to understand why the gossiper and listener suffer, since they participated in the sin.  But why should the subject of the gossip suffer?  After all, he was not even present at the time!

The answer lies in the fact  that speech is a revelation of something that was previously hidden (in thought).  Therefore, by speaking badly about another person it actually causes that person’s bad traits to be more pronounced in the world, which could lead him to be the subject of a heavenly decree of punishment.  Thus, it is bad speech that can harm another (and not thoughts, that remain hidden), which teaches us how careful a person should be with the words he utters about another.  ~ The Lubvaticher Rebbe based on Likutei Sichos, vol 15, p 31ff