* Not having the chance to go to a party. (i.e. never being invited, or being in government-mandated house arrest for years to accomodate for other people who might die of a certain disease. Or both.)
* Having the chance to go to a party, but still refusing because of the dread going to a party induces. (Possibly related to trauma caused by previously going to a party, see below. (Or any trauma caused by other humans, really.))
* Actually going to a party, but not having the communication skills to naturally start a conversation with some random person.
* Starting a conversation nevertheless, only to come to the same conclusion (also traumatizing yourself in the process after you get ignored).
* As an extension: trying to join a conversation, but being ignored by all participants for being uninteresting, annoying, weird, or simply ugly. (Or all of the former.)
* Sitting/standing alone for most of the party because of the aforementioned problems, then after a few occasions deciding that surely this wasn't worth the time and never going to a party again.
Alcohol may or may not help with these problems. (It doesn't for me, so in the end I just gave up.)