I don't know what that is actually, but some say it could be from depression. Depression is a complicated thing, some people can get so affected by depression that they just can't cry. Do you get significantly anxious and/or depressed a lot of the time?
My mum is a perfect example. She's always had depression (probably me being born as someone with Asperger's never helped), and lots of things have gone wrong in her life, and she is also a born worrier, and now she has got to a point where she can't cry at all. She always used to cry quite a lot, but I haven't heard her cry for a few years now, and she said herself that she just can't cry any more (unless something really devastating happened, like the loss of a relative or something, God Forbid).
I do cry a lot. I get tearful over the slightest thing. Even if somebody at work is telling me off in the nicest possible way, I feel I am fighting back tears as if to say ''why can't you just leave me alone, I'm just here to do my work!'', even though they're not exactly telling me off as such. I sometimes wish I could become someone that can't cry so easily.
That's the only answer I can think of, it's probably not very accurate but it's the best I can come up with. Stay tuned for better answers others may give you here.
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Female