Some people may be offended by the subject matter to follow.
I was reading about a study done on orgasms in both women and men. The participants were given a radioactive oxygen tracer and then had their partner stimulate them to orgasm within two minutes (short half life of the tracer). The participant is put in a PET scanner during this to watch where the blood goes in the brain during orgasm. The results were interesting.
First off, in both genders dopamine is released and floods the brain from the brain stem. Oxytocin is also released if the buildup is slow enough. This combination creates an attachment to the partner. Oxytocin being the 'cuddle drug' because it brings feelings of love and dopamine being the 'addiction drug' as it is associated with the reward center of the brain.
When a woman orgasms, her whole brain is flooded with blood. She can actually lose herself completely in the feelings and even pass out from pleasure (kinda rare, though). Because her entire brain is activated it is even possible for a woman to think her way to orgasm. When she imagines someone stroking a part of her body, her brain figures that's what is actually happening and reacts as such.
When a man orgasms, his brain is also flooded with blood...except the part responsible for awareness. This is probably because his ancestors had to remain alert for predators or danger even during sexy time. This could be why men are considered to be more visual and prefer some imagery to get things moving along instead of simply his imagination.
Not to say women don't like imagery as well, we do, but our bodies react differently to what our brains are seeing. Women's bodies react as if it is actually happening to them where men will become aroused but unless someone is actually touching him, his body simply doesn't transfer the physical sensation from body to brain.
This difference in orgasm could also explain why a man is generally able to get to the finish line regardless of his surroundings. And why a woman can feel like the finish line has moved farther away because she is too aware of her surroundings to relax and enjoy the ride.
And so I wonder as well if this is why nocturnal emissions in men are wonderfully pleasurable. When the man is sleeping his awareness center is not as engaged, allowing his imagination to transfer sensations to his body. Women also have wet dreams but the dreams are similar in pleasure factor to the sensations when awake.
All of this could also be why many people use sex (with or without a partner) as stress relief. The oxytocin/dopamine mix is similar to a cocaine hit and temporarily relieves negative feelings. I can also see how this might contribute to sexual addiction in some cases in the same way that some people will become addicted to cocaine while others simply don't.