A year ago, I started to get some rashes in my mouth and tongue (I have root canal treatment done) and my dentist said it is lichen planus and there is no treatment for this condition. He also said it might stay for few years and then will disappear by itself. Now I have noticed that these rashes has almost gone, but slight discoloration (light black) on the tip of my tongue and one or two red spots on the tongue remains. What is this lichen planus ? Is there any treatment for this ?

Summarized question: What causes lichen planus?