I’m not entirely sure we should believe that humans are the only species to have the pain-pleasure dynamic. When certain large cats (panthers, lions) mate, the male bites the neck of the female and this often induces the female to ovulate. Bonobos have been observed in play that appears to include elements of mild pain. It’s rare for any characteristic to spring fully-formed in a single species; evolution works by degrees. Even supposedly unique human speech has clear antecedents in primate grunts, howls, chattering, etc. and likewise the vocalization of cetaceans shows sophisticated communication patterns. Given that all animals have pain receptors and all animals can release endorphins when stimulated, it’s likely that other species share our reactions — they may simply not often have the ability to induce with the ease that we do.