In the original source novel; The Hellbound Heart, the text offers a number of reasons why someone would complete the puzzle box:
- It's intellectually stimulating
- The box makes a series of pleasant tones when each section is completed
- The box offers a gateway to an alternate dimension where untold pleasures are promised
Systematically, she began to feel her way over the sides, testing her
hypothesis by pushing and pulling once more. The cracks offered her a
general geography of the toy; without them she might have wandered the
six sides forever. But the options were significantly reduced by the
clues she'd found; there were only so many ways the box could be made
to come apart.
After a time, her patience was rewarded. A click, and suddenly one of
the compartments was sliding out from beside its lacquered neighbors.
Within, there was beauty. Polished surfaces which scintillated like
the finest mother-of-pearl, colored shadows seeming to move in the
gloss.
And there was music too; a simple tune emerged from the box, played on
a mechanism that she could not yet see. Enchanted, she delved further.
Though one piece had been removed, the rest did not come readily. Each
segment presented a fresh challenge to fingers and mind, the victories
rewarded with a further filigree added to the tune.
and
"Oh yes. We know Frank."
"He solved the box too, am I right?"
"He wanted pleasure, until we gave it to him. Then he squirmed."
and
"You did it in ignorance," the visitor said. "Am I right?"
"Yes."
"It's happened before," came the reply. "But there's no help for it. No way to seal the Schism, until we take what's ours..."