I think the connection between these things for some subs is in the brain chemistry and stimulating dopamine releases, which gives you that euphoric feeling, and occurs with all these activities as well as psin. However, and I hate to play into stereotypes here, some painsluts are working out childhood issues rather than it being about using dopamine to access subspace and/or euphoria.

Personally, I hate pain... I accept it and therefore submit to a Domme's will by doing so but I freaking hate the sensation itself. However, I really love roller coasters (I have actually taken Dommes to both Cedar Point and Six Flags Magic Mountain on completely non-kinky day trips) as well as spicy foods that taste good (although I have no interest in hot challenges).

I guess my point is that there is a casual link between these things and you can find that chemical basis, but you can also ask about punk/metal music or goth/Halloween culture and find the same linkages.