Question

Why the glans penis is so sensitive?

Answer 1

The glans penis has special receptors which make this area very sensitive. Additionally, the skin over the glans is devoid of keratin which is a tough layer which reduces sensitivity of normal skin. that is also the reason that this skin is pinker than normal skin.
After circumcision, this skin of the glans is constantly exposed and therefore nature makes this skin keratinised (which is theoretically less sensitive) and becomes browner in colour.
Pulling back the foreskin is necessary as a hygienic measure to keep the portion between the glans and the foreskin clean. this is an important hygienic measure to avoid various diseases.