I was reading a book the Ming court that was discussing some of the emperors eunuchs and I was wondering where these men would have come from. I know that castration was a punishment meted out to some famous courtiers like Sima Qian but it wouldn't make much sense to surround the emperor with convicted criminals. So, were these some kind of slaves purchased as children or people punished for their family's actions? Did people become eunuchs voluntarily, and if so why, when their would be conceivably less awful ways of moving up in the world.
Feel free to discuss other Imperial courts besides the Ming, from what I understand eunuchs were more or less a feature of all of the dynasties.
Ming is the "golden age" of Chinese eunuchs, that's the period with the most of them running around, so you have made a solid choice! One of the few good English texts on Chinese eunuchs is just about this period.
Eunuchs of this period would have been majority voluntary or semi-voluntary, both adult men who elected to be castrated to join the palace service, and some would be castrated under the auspices of their parents as children for service.
As to why... I know it's insane to think about now. But (assuming you're male, and that you've had sufficient education to type this post, and sufficient privileges in life to find and use a computer) it's just hard for you to imagine this as a very do-able sacrifice for a better life. You have to think outside of your situation a bit. If you were a poor farmer with many sons, castrating 1 or 2 and sending then to the city to find service in a royal family or (with luck!) inside the palace is a reasonable gamble, considering. And if you're an adult, I mean, wouldn't you be tempted if you could be super rich and powerful? It is a more accessible option to a poor and uneducated man than the Imperial examination, which required extensive education. Consider very high ranking eunuchs would outstrip in rank just about everyone, your "career" options are virtually unlimited once you get in the palace, and even if you were just sweeping corridors you would be given food, clothes, money, medicine, and if you were promising, education. If you were an adult man with a wife and children (which did happen) you could send money home. If you were an adult child of poor farmers you could also send money home. Castration is a reasonable choice, which you have few others.
Other than the book I linked above, which is certainly decent but the author doesn't like eunuchs much so I'm not fond if it, there a very interesting article about Jesuit missionaries observing Chinese eunuchs in the Late Ming period.