Oh my gosh, my cat does this often! It drives me crazy because he leaves big wet spots on the couch. He just licks and licks the same spot over and over. My cat used to suck his tail too......and occassionally he'll do it now. I'm not sure what brings it on but I think he first started doing it because he was abandoned as a baby and probably feels comforted by it. When we got him his eyes were still closed and I had to bottle feed him.