I do know that the first time I heard it was from a woman about 10 years younger than myself who was such a person. Over the years I have noticed that almost everyone I have ever heard casting this verbal mortar at anyone is not a very nice person.
Many of them are relentless optimists who think every cause for a "negative"emotion is "a pity party." What I have decided is that most of them are running like mad from knowing themselves or examining their life. None of them have, to my knowledge, taken any responsibility for the downsides of anything they do, whether it is stepping on a long time friend on their way to the top nor crushing their child's self-esteem. A few have been, in my opinion, genuine sociopaths, many asocial, and almost every one of them a callous, arrogant moving box of insincerity and emotional stuntedness. Without exception they were willing to use, abuse, exploit and mistreat others and then accuse the people they hurt of having a pity party when their victims object.
I have seen people cast the term at people suffering from clinical depression, including myself. Hey! guess what depression is scary, particularly to people who always believed they could do anything they set their mind to. To me it often felt like being in one of these commercial size washers on power wash: no control over anything, getting your head bumped constantly, feeling like you are suffocating/drowning, and nobody on the other side can hear me calling out for help. When someone finally does open the door, they curse you out for hogging the washer (tell you to get over your pity party). Then they slam the door and walk off.
I have seen people cast the term at someone who just gone through a series of life's wallops in a short time and it lost and overwhelmed. People who are suffering from PTSD, which you really don't have to go to war to get. The long term unemployed who are fighting depression, money woes, often health woes, and hearing themselves described as "lazy," and struggling with urge to rip off that person''s neck and shit down his neck.
What I have never seen is the use of the phrase produce any positive direction for the person it is applied to, and often a negative one. Nor have I ever seen the person using growing in respect from the objective observers.
I hope none of them have ended up in "helping" professions such as psychology, healthcare, or parenthood. The phrase "pity party" doesn't help anyone, including the person who uses it. It just a rationalization in your own hear for being an asshole who doesn't really care but was willing to pretend to be a friend until your emotional attention span expired. It's a sign you are running from something deep inside yourself that you are too chicken to even look at face-on