Why do I bite my nails/lip/cheeks?

It could be a sign of anxiety or stress. When I was little I knew this girl who always bit her nails up to the point that they bled, and she eventually got taken to a doctor who prescribed this weird stuff she put on her fingernails. So every time she went to bite them, it tasted really bad and I think she eventually stopped.
Of course if your habit is due to stress or anxiety, that’s something serious that should be looked into. I’d recommend seeing a doctor about this, even if your habit does minimal or no damage. It may be cause for something bigger that needs to get treated, and he or she can give you more resources and options. Because I’m pretty sure that if this is constant and ongoing, it is side effects of something like I mentioned.
And if not, a doctor may still know what to do about it if this truly bothers you, and you want to put an end to it.

There a couple of reasons why you cheek bite. But since it is combined with nail biting the most likely cause is anxiety.
Best not to obsess about it. Just simply remind yourself to stop when you do it. Replacing habits with others like chewing on gum or a tooth pick should also help.

Could be a simple habit without much psychological explanation.

But, it also could be a crutch to distract you from your thoughts. When you do wither of the behaviors try to focus on what you are thinking about, or what you are avoiding thinking about. Are you anxious? Sad? Worried? Bored? Once you figure out what emotion you are avoiding you can work on changing the habit.

This is called an oral fixation, and many people have one. It is a deep-seated psychological quirk, but not one to worry about too much. More like just another thing that makes you you. The fixation can lead to bad habits like you describe, and can also contribute to bad habits like thumb sucking in kids and smoking cigarettes in adults. If you’d like to learn more about oral fixation, this Wiki page is a good place to start.

And trust me – the first time you impact a saliva gland in your cheek from chewing, you will become very aware of this habit and do everything you can to quit immediately.

Try tea tree toothpicks to give your mouth something else to do – these are also available at Whole Foods & Trader Joe’s and most health stores.

Maybe you’re just really hungry??

You bite your nails/lip/cheeks because you have an innate longing to live as a wild and free hunter gatherer. Modern civilisation is crushing your soul and alienating you from the beauty of nature and the utter freedom that your primitive ancestors enjoyed. Your suppressed desires to live free and close to nature manifest themselves as the compulsive and neurotic reaction of nail/lip/chee biting.

I recommend a camping trip.