Someone who just can't get enough.
An incredibly high threshold for either or both physical and emotional pain, coupled with great stubborness (or determination, as they call it) makes someone a sucker for punishment.
A sucker for punishment Is strongly self reliant and independant and take it apon themselves, and no-one else to solve their problems. Their relentless determination and unbreakable spirit means they never back down, never give up, they are absolutely determined to keep going despite what pain it may be causing them, and what obstacles stand in their way. The person is desperate to get things done by themselves, they usually reject help until they literally can't carry on anymore.
To others their efforts may not seem worth it, but for them the victory alone is worth it,never mind the prize, they gain great satisfaction from achieving their goals but failure from resorting to someone elses' help.
Their relentless determination is usually a good a thing, such people rarely fail, but is also often a bad thing.
If they ever can't succeed for whatever reason, they will just try and try and try and try until their apparently never-ending strength has been spent, this is very painful and exhausting to both them and their friends who they commonly reject the aid of.
1:"Damn, that guy's fell off like hundred times, he's bleeding and everything!"
2:"Such a sucker for punishment, when will he learn?"