This is either one of the most popular reasons for castration, or else maybe there is just more transsexuals that talk about castration?   Anyway it's a common reasons.  In many cases it's a first step toward sex change surgery. 

Anyway there isn't much I can say here.  A transsexual gets castrated because they don't want to be male.

I call this Gender Correction - that's probably not quite true, but it is a close I as I can come with a word or two.

I believe the idea here is that by being castrated, you become a new person ... not male and not female. It's like calling An Eunuch a gender of it's own. Which is probably what it should be.

This seems to be a very common reason ... men who dislike being enslaved to a sex drive, seeking to control it instead of being controlled by it. It is what was my primary motivation in 2001.

There are many men who enjoy various types of ball pain and/or torture (I did) and of course that runs the risk of permanent damage to the testicles.  Other men enjoy castration-play, where a partner threatens castration, but doesn't really do it. Sometimes.  I suspect some men have been tricked by a partner who castrated them without approval, but I know of no one like that.

Anyway any type of sex-game that abuses or threatens the testicles does of course have the risk of accidental castration. 

I won't even pretend to understand the thoughts behind this.  I do know it's common for the castration to be the slaves 'gift' to his master, or a symbol of servitude.  You might expect this to be common in the S&M community, but I have been told by some slaves that have written me that it isn't that common.  Anyway it is a valid reason for people into S&M.

It's better to give pleasure, than to receive pleasure.  They're called subs or bottoms.  I think the majority here are heterosexual men who are married.  I have tremendous respect for these people because it takes a special person to realize the emotional value of sex is more important than the physical value.   This seems to be more common among straight people than gay people.