The origin of the mutilation was to keep boys from masturbating. It makes it more difficult to masturbate. The recent practice of it has been largely a revenue generating exercise of the medical community. Most doctors these days are no longer doing it, or at least advising strongly against it. The myth of being cleaner is just that, a myth. Boys can all learn to wash properly. And if there was any validity to the myth, why wouldn't the female labia also be removed routinely. It constitutes a very similar situation. I am not in any way opposed to a male being circumcised, but I am adamantly opposed to what constitutes male mutilation when it is done too babies. If one is too be circumcised, it should wait until the individual is old enough to decide for himself that he wants it.
It is usually because of culture. In some cultures like Filipinos for example, a male is not a man and is shunned and teased if if he is not circumcised by the time he reaches puberty. And this goes on even to this modern day and age. The jews do it because God told them so in the old testament. Others do it for hygiene.
Here are the reasons I remember having heard so far: better hygiene, culture, tradition, religion, better looks, parent's decision, medical reasons, health, better feeling, girl's preference, less masturbation.
There's always a big debate on the whole issue and you can be either for or against it. I'm uncircumcised myself and have chosen to remain so as long as there are no problems. But I have to admit that I have seriously thought about the pros and cons of getting cut.
For most guys it's decided for them at birth. I myself am not circumcised but have thought about getting it done in the past. I have thought about getting it done because I wanted to fit in. I'm not sure what country you're in but here in the United States it's more common to find a guy who is circumcised. Because of that it seems that most women here think that it's nasty or weird. Lately though I have come to the decision that I'm fine the way I am and if a woman doesn't wanna be with me because of it then so be it. To me the benefits of not being circumcised out weighs the cons.