Yeah it's pretty simple really - some people don't have a lot of natural endorphins. For example, if I do some sports, I don't get the endorphin rush that a "normal" person may get. It reminds me, I remember watching a documentary about people who do suspension by piercings and the guy said that when most people do it they get this great almost orgasmic feeling, which masks any pain, but some people just get all pain and no pleasure and hate it. However, note that having low endorphins does not suppose a cause - it's not like that means it's some genetic defect that is untreatable and all you can do is be on opioids forever. It's not a "diagnosis" like a disease, it's just a state of endorphin levels - more like a symptom. Although I'm sure it will probably become it's own disorder eventually so Big Pharma can cash in on it.
Sorry about continuing the off-topic branch of the thread but I wanted to chime in here as it sounded kind of dismissive for people to say "where did you come up with that?" or assume that a quick cure for it was sport - it's really not that simple. For example, I find it exceedingly difficult to get any motivation to even do things like sports/exercise, there is little reward when I do, and it feels painful to me. However I try not to think of myself as "endorphin deficient" as that seems like a disempowering and limiting label. But I do have to be kind to myself and understand that I shouldn't be too hard on myself because I feel things differently from others around me.