It’s vanity.
Look at me, see how extreme I am!
I was part of this culture over 10 years ago. I never got into this sort of pain/pleasure exercise, but I'd seen it done plenty of times.
Most body modders I knew were "straight edge" (AKA sober). They were teetotalers and often adamantly anti-drug. Their way of "getting high" was through the body's natural response to painful stimuli. In this case, the initial pinch of the meathooks was unpleasant, but once the adrenaline and other brain chemicals started reacting, the high was apparently quite pleasant.
I watched several people, male and female, suspend from hooks and some even would bounce around the room like a trapeze artist. Craziest thing about it: there was very little blood. Say what you want about these people, but I personally knew men and women, professionals, college students, even a politician who was into this. The vast majority were honest, hard-working American men and women who simply had an alternative way of getting high.
Wasn't for everybody, but these people were "normal" outside of this.