Don't you know anyone that donates money? Or that volunteers their time? Or that doesn't use all their deductions when doing taxes? Haven't you read stories about people who anonymously donate kidneys? There's so many examples - and the only way for your world view to "work" is if all of them only do these things for selfish reasons.
At some point one has to acknowledge and believe in good and cooperation and decide if they mostly want to try and operate in coop mode or in selfish mode, but the more you believe others are likely to choose coop, the more like you are to do the same. So you need to start from the belief that good and coop are things other people will also choose. Your world view prevents this "from the start".
Of course, it's impossible to predict complications. Some donors have died on the operating table. The risk of complication is very low, though.
If you have any other questions I'm happy to answer.
Sources
* https://www.livingdonorgames.org/
* https://www.kidneyregistry.org/for-donors/kidney-donation-bl...
* https://www.kidneyregistry.org/for-donors/i-want-to-learn-mo...
come the f on u for real?
There's more information about living kidney donation here: http://waitlistzero.org/living-donation/risks/
Donating money? Volunteering? Even donating a kidney can all categorically be understood in terms of individual gain.