One of the most common reasons is stress. If you think about it, this makes a lot of sense. [i admit i bite my nails when i'm stressed (namely during graded recitations and class performance etc.) but still. i bite it even when i'm not]Believe it or not, the second most common reason for biting your nails is boredom. You have nothing else to do, so your fingers head for your mouth. [PROVEN!! everytime i watch the tv. my hands would automatically head for my mouth without me even noticing]
Another reason could be simply that you are suffering from a mineral imbalance. There is even a name for this condition. It is referred to as Pica and is sometimes connected to nail biting. If you aren't taking a multi-vitamin, this might be a very good reason to start. [i take vitamins everyday so.. i think not this one]
This might sound a little strange, but I know from experience that former nail biters often revert back to old practices when their nails get too long. Instead of reaching for a pair of clippers to bring the nails back to a manageable length, they bite them off. [ahaha. sheesh. are these people psychic??]
Excessive nail biting has also been traced to dieting. If it is hours until your next meal, I can understand how this could come into play. It makes sense to change your method of dieting. As a newly diagnosed diabetic, I now eat six times a day and my nails have never looked better. [huh?]
stuffs like insecurity & pagkukulang sa pag-breastfeed are reasons i have heard from others. due to this "research" XDDD i was able to obtain 3 reasons while i bite my nails.. i know. THERE'S MORE. ahaha.
FOR FELLOW NAIL BITERS LIKE ME.
IF YOU ARE AS DETERMINED AS ME TO PUT AN END TO SUCH HABIT. CHECK OUT THIS SITE. AND FOLLOW THEIR SUGGESTIONS.
http://beauty.about.com/od/nailbiting/qt/stillbiting.htm