I chew my fingernails and pick at my toenails. I remember that when I was very young my Mom tried to stop that habit by painting my nails with nail gloss or something like that. Ugh, that stuff tasted nasty! But, you get used to it, lol!
In my early teens I realized that it must be natural. I figured that it was instinct. How do you suppose that our ancestors did it before the invention of files and clippers? So it doesn't bother me all that much.
The bacteria that I am exposed to helps strengthen my immune system I figure, because I rarely get sick.(knock on keyboard)
I watched Lord of the Rings recently and noticed a close up shot of one of Frodo's hands. His nails were all chewed up just like mine. So it turns out that Hobbits chew their nails too. At least some of them do. Which is cool.