According to Taylor et al. (1996), the circumcised penis is missing an average of 51% of the shaft skin.  The authors stated, "Skin and mucosa sufficient to cover the shaft was frequently missing from the circumcised penis."
When boys start puberty, they start to get erections due to the sexual hormones. The normal, intact penis provides for this by having a double layered foreskin that unfolds to provide sufficient skin to allow the shaft of the penis to expand in length and width while erect.
A boy who has been circumcised by the American standard technique of removal of sufficient skin to cause a tightly stretched, unmovable foreskin is likely to have painful erections if there is insufficient skin to allow full expansion of the shaft of the penis.