The science of sex is still evolving. Not all these writings have scientific backing! But my thoughts! The brain has chemical pathways it follows to achieve an orgasm. Or the paths that are followed result in an orgasm. I'm not sure we're sure.
Orgasm, we know for a fact, s cause a release of the brain's opiods, serotonin, prolactin and oxytoxin. In fact the seratonin release is the turn off signal. It is also backed by research that the seratonin release then immediately reduces the dopamine presence in the brain. Dopamine is released during arousal. So the dopamine that was perceived by the brain to be a turn on, decreases at brain sites, and this leads to a resolution of the response.
Thus, I assume, the chemicals are released, in what sequence, in what amounts determines our unique ability to have an orgasm, and perhaps why sex with one partner is different than another partner. We assume that if someone is very aroused, then there's a lot of dopamine in the brain, and just a little seratonin release (the first orgasm) may just not be enough to make us want to quit.
On the other hand people who maintain high levels of arousal: the 24/7 dopamine releasers, perhaps they become a bit desensitized to the minor elevations in dopamine levels that just a bit of "same old routine" sex presents. Perhaps explaining why some need more intense experiences to then get aroused.
As we try to find chemical stimulants for desire and arousal one of the puzzle mysteries is how to weave a chemical feeling into a human response that can only be appropriate in some contexts. And perhaps giving someone an elevated level of chemicals, but not a varied level of these chemicals is going about the fix-it routine a bit longer.