At first, you might think that there really isn't a "why" when it comes to nail biting. You just started doing it one day and kept on doing it until it became a habit. That might be true for some nail biters. However, for many of them, there are very specific reasons for it, such as boredom, stress, fear or anxiety.
Maybe the first time you bit your nails was also the first day of school or the first time you attended a sleepover at a friend's house. Biting your nails was something physical that you could do to relieve the stress, fear and worry that you were probably feeling.
If you can figure out the motivation for biting your nails, then you can find a more constructive way to deal with your feelings. Consider keeping a sort of nail biting journal. You don't even have to write much; just answer a few questions when you find yourself starting to bite. Where are you? What are you doing? How do you feel, both physically and mentally? If you keep up this journal, you'll be able to look back and see a pattern.
Now that you see why you're biting, find ways to deal with it. You won't always be able to remove the source of stress, so find something else to do. If you feel bored at work, go for a quick walk or get a drink of water. If flying makes you anxious, make sure you have lots of reading material and a laptop so you can watch movies (or go online). Talking to somebody about your feelings can also help.
Maybe you've tried all of the suggestions we've given so far and nothing has worked. Next up, it might be time to seek professional help.