Ticklish?

I was tickling one of my kids earlier and certain ideas popped into my head: I have known people who are completely non-ticklish. I know what it is like for them, since most physical sensations don't tickle, but they don't know what it is like to be tickeled. I can't think of any concrete disadvantage from not being able to have this sensation and one might wonder if there is any evolutionary advantage to being ticklish. It is a nice sensation though,-if taken in moderation and as hard to describe as color to the colorblind. If I were to lack some sensation, I think itching would be a good one to not have to feel. Of course itching is unpleasant, but it is awful nice to have an itch scratched. It always feels best when it is someone else doing the scratching. That is another odd thing about the tickle--it has to be someone else who is doing the tickling or it doesn't work.