how to into emotions

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I was over at my mom's house and she found out on the phone that her friend, who has had cancer for a long time, is now cancer-free. She came and told me the news all excited, then promptly become offended because, in her words, I "wasn't reacting." She said I just stared at her blankly and said "oh that's great" in a robotic way. She now says she feels bad instead of good since I "ruined the moment" and accuses me of being displeased that she was happy. She has accused me of being "zombie-like" or robotic many times before, and always takes offense to it, as if I am deliberately acting that way toward her. That's not true at all, though she doesn't believe me when I tell her that.
I've been accused of being unemotional and "robotic" by other people before. I also haven't had friends or a social life for years now. I'm 27.
I actually can be very emotional but only when I'm alone, and usually about weird, eccentric stuff. Like tragic anime. Or airplane crashes. Or thinking about soldiers dying in war.
I'm generally satisfied with life on my own terms, but obviously this emotion stuff gives me difficulty with other people, like my mom. Wat do.