hello
I believe in part this to be inherent . My biological family has many nail biters in it. Additionally, as a young person (and even now when nervous) I pick at my fingers. I have read articles where nail picking/biting can be equated to the same type of anxiety as those who pull out their hair (Trichotillomania) sounds strange doesn't it, but true.. Also, some might say.. ah stop picking at your nails, you can do it.. true.. you can.. but when it's something you've done from such a young age, it becomes very difficult to stop. My mom used to get so upset with me when I would start in, but I often did it without realizing I was doing it. Not to mention, I truly believe it was a built in mechanism for me that I used to fight anxiety or stress.. Now, as an adult, I can manage better, however I will say, stress can still bring on a bout of picking at my fingers as can different foods such as too much caffeine or even sugar. I try to limit both.
Exercising has also helped me to not pick as much.