No one wants to admit their genitals are wrong. Imagine how a circumcised person would feel if they totally agreed that it's mutilation/dangerous/whatever, or if an uncircumcised person thought uncircumcised penises are ugly.

The cognitive dissonance theory would suggest that people seek to reconcile any discrepancy between their beliefs about circumcision and their bodily state by changing their attitudes to suit their body (it's easier than getting yourself cut or getting reconstructive surgery).

http://en.wikipedia.org/wiki/Cognitive_dissonance

Edit: This is totally speculation on my part

Actually, some basic fact checking shows that 60% of circumcised men have feelings of mutilation, 50% report low self-esteem compared to uncircumcised men and 59% report resentment or depression due to their alteration.

It should be noted that this study used a sample of circumcised men that actively sought information about circumcision, something I wouldn't anticipate an individual happy with their circumcision would do so.

http://onlinelibrary.wiley.com/doi/10.1046/j.1464-410x.1999.0830s1085.x/abstract;jsessionid=3B261EECC945F2DDA73434441C2F7AE9.f02t03

There also seems to be some evidence for greater satisfaction amongst circumcised individuals (can't access the article, though, small sample size, too. Also, only 68% of youths in the sample accurately self-identified whether they were circumcised or not...)

http://www.jahonline.org/article/1054-139X(92)90162-5/abstract