Why are so many girls not ticklish?

I'm very ticklish. I know a few girls who gratefully take advantage of that. I don't mind the fact that they tickle me that much (although it can be a bit annoying), but most of them I can't tickle back! Then it turns out that they are not ticklish and they tickle me even worse.

How do you handle it so well and why do you enjoy doing it so much?

Why are so many girls not ticklish?
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