Nice quote. Does the author tell why he ends up hating a human when he loves humanity. I think I too have that problem. I look at all the TED talks and think I should love humanity but I have a bad husband to my wife and a bad father to my kids. tia.
I think it's not anything wrong with you per se, nor have I read the book, but I think it's because like the author says, dealing with people on an individual basis requires personal sacrifice. Particularly, emotional sacrifice. Allowing people to get a cold without letting it bother you. Making compromises and being forced to do things you don't want, while not being able to do what you want, on a daily micro level.
Whereas tasks that show your love for humanity are more detached from actual individuals. Curing cancer requires you to interact with lab equipment and microorganisms more than humans. Sure your opportunity costs here may be playing video games or traveling, but they are less emotional costs and more material costs.
Paying emotional tolls is difficult, and from my experience, many people are simply incapable of paying them. I mean, they are or may be capable, but they would have to change the way they act or behave, and they are not willing to do so. I think there's a genetic component to it.
It is nice to know you are fan of David Foster Wallace. I first came across This is Water about 10 years back, shortly after he passed away. It was an article that appeared on New York Times. The article described what an extraordinary writer he was. The I read This is Water. It was jaw dropping. I have read that essay several times after that. I have remembered the contents of that article several times when struck by melancholy. I was deeply moved by that essay. He has written This is water with such deep understanding of the human condition and with such empathy. I have wept for his untimely death. I always wonder, how does he was able everything about the humans in slow motion and with such high resolution. Amazing. No words. We are worse off without him.