These people barely get past the age of 30 because of the fact that by not feeling pain, they can get seriously injured and either 1. not realize it, or 2. lack the reaction to save themselves.
Think of every time you've touched something hot and immediately pulled your hand away without even thinking.
Now imagine not pulling your hand away from the surface that is scalding your skin off
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http://28.media.tumblr.com/tumblr_lcb35gGx0t1qailr4o1_500.gif
http://www.megavideo.com/?v=57N0YAEJ