From: Seattle, WA
In my experience, this depends on the masochist.
I do not like surprises. At all. And there's certain pain that I will retaliate for swiftly: there's no endorphin high, there's no sub space, there's simply the awareness that I've been attacked. Now, during a negotiated scene, or in a negotiated relationship where I'm already prepared to bottom or braced for the possibility? That's different, and negotiations with a trustworthy top means that certain experiences are off the table. That's just me, though.
I know one person who was strolling around a grocery store during the summer, suddenly wondering why their toe felt so good. They looked down and realized that their big toenail had ripped, and all they were getting from it was endorphin happiness. This person was also confused during a conversation I had with someone else about body art, during which we both shuddered at the idea of getting a brand and having to deal with that large of a burn. They'd apparently never suffered anything unpleasant upon experiencing a burn. Or slamming their hand in a car door, another example through which they'd just shrugged.
An entirely different person is an experience junkie: if they haven't already done it, it's probably on their bucket list. Previously part of TNG ("the next generation" of local kink), this person was on a mission. They eventually trained themself to learn to eroticize pretty much anything, and their go to example was eroticizing dental visits ... which they used to fear. This person since started teaching class within the local kink community.
So, I'd advise against lumping masochists into a homogeneous group. There is a lot of variation in how people process different types of pain.