Why do people chew on their nails and how can this stop?

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Nervousness, fustration, stress, these all lead to nail chewing. Many people have this bad habit of nail chewing, i myself fall for it as well. it's just like my bad habit, i've never had long nails in my life:( i sure do want some though. so if you can help me out, just give me some advice :) please and thank you!