I had horizontal ridges on my thumbnails and did not know why.  Thought it was possibly a fungus.  I went to the dermatologist, who told me, "Stop touching your thumbs.  Let the manicurist take off the cuticle, not you."  I said, "I don't!"  He asked me if I get dry calves, elbows, and hands, and I said, "Not particularly," but then I did notice that those areas of my skin were a bit dry.  As I left the office, I impulsively curled in my pointer fingers to rub off my thumbnail cuticle.  That is when I realized that I WAS in fact rubbing out my own cuticle and messing with the nails.  I vowed to catch myself in this impulse and not touch them, and turns out in a few months the thumbnails grew in smooth and normal.