Yeah, I was circumcised as a baby, so I really *don't* know what I may or may not be missing, but what I have read is that the foreskin is not "just a flap of skin".
The head of the penis is similar to the tip of the female clitoris (in a fetus the clitoris and penis *are* the same thing) both are very sensitive nerve rich mucous membranes covered in a "hood" of equally nerve rich skin which keeps them relativily moist and protected from uncomfortable overstimulation.
But when you cutoff the foreskin - which has the most nerves per millimeter of any other part of the penis (including specialized nerves sensitive to rolling and stretching) - you not only remove all of it's sensitive nerves, but you also expose the extremely sensitive head of the penis which, in response, dries, toughens (becomes "keritanized") and loses it's ultra-sensitivity. (It has to desensitize to compensate, otherwise it would be so over-sensitive that something like wearing underwear would be uncomfortably over-stimulating - painful actually.)
Plus circumcision removes the majority of the frenulum - the y-shaped bridge of skin that attaches the fireskin to the head of the penis. This is probably the single most nerve rich and pleasure sensitive skin on the entire penis, and it's just cut away and chucked in the bin.
So a circumcised man can stretch his penis' existing skin, over time, to again cover the head of his penis, and in time the head will regain some of it's lost sensitivity, but the new faux-foreskin is never going to have the same nerve density as the real foreskin tissue had - and no amount of stretching will make the hyper-pleasure sensitive frenulum magically regrow.
