I believe that circumcision should only be done when an informed decision can be made, i.e. when the child/man can fully understand the risks/possible outcome. As many others have said, the foreskin keeps the glans moisturised and protected, it has a function, it's not just a flap of skin.
The only reason for circumcision should be medical need.
On the topic of medical need, both my DS's had phimosis, and the opening of the foreskin was extremely tight, could not be retracted at all to clean and would balloon when they had a wee. There was a risk of infection and other issues so the consultant recommended circumcision. I provisionally agreed, and then went home to do some research, which is when I learnt about the potential negatives of circumcision (obviously a lot of boys/men are absolutely fine with it).
I read about preputioplasty, where a slit is cut into the foreskin so that it can be retracted, but the foreskin remains intact, doing it's job. The consultant wasn't very keen on doing this, but he agreed. He wasn't keen because of potential scar tissue and the chance of the slit re-healing and them needing circumcision down the line.
10 years on, my DS's say that everything is fine, looks fine, and works normally.
Just mentioning this as an alternative to circumcision for medical need, bit off topic I know.