What happens to the penis nerve endings during intercourse? why such pleasure ?

Krause's end bulbs: Special nerve endings in the genitals that transmit directly to the pleasure centers in the brain. Thanks for trusting HealthTap!

Hooked up that way.: input goes to particular parts of the brain deep in that release dopamine and wired up to feel pleasure. The addiction researchers tell me in susceptible individuals these parts of the brain are even more stimulated by stimulants (but may permanently damage those areas so pleasure from normal sex is damped down.. So Not Worth It).

Sensitive: The penis, when in an erect state is hypersensitive including the nerve fibers at the end of the Glans. If hypersensitive and premature ejaculation is a concern, using condoms may help decrease the sensitivity slightly and prolong the ejaculatory process.

Pleasure detectors.: The nerves in our body have different functions. They report pain, hot, cold, pressure, and pleasure. Why does it feel good in particular? It was probably evolution. Because if sex hurt nobody would do and the human race would vanish.

Nothing happens: Not sure what you mean by asking "What happens to the penile nerve endings...." Nothing....they don't go anywhere, they don't do anything except transmit signals related to the tactile and pressure sensations experienced by the penis. Why the pleasure? It happens to be based on how that part of the brain where these signals are processed reacts and "tells you" what you feel.