you're fulfilling a basic need that people dont recognize: human contact, be it physical or whatever.
well, therein lies the introvert's paradox. the introvert knows they have this need, but cannot believe that they can meaningfully contribute in a social engagement -- because they don't (or possibly CAN'T) understand that everyone wants them there because THEY ALSO NEED PEOPLE. particularly self-negative introverts spend a lot of time in their heads calculating some scalar component of self-worth relative to those around them, and if the headmath doesn't add up ("i can't do small talk so people will not like me," "i don't really drink so people will think i am uncool," "i don't have any special qualities that people would care about"), they continue to hover on the fringes, caught in a twilight between loneliness and social engagement. the only way to break out is through cognitive therapy (well, or heroin), but most introverts know themselves so well they already have plans to fight therapy, since on some level they enjoy the strong self-parameterization that comes with holding one's self at painful remove from the rest of us, like it has an innate nobility or a meaningful ache.
and for some, therapy is almost impossible: they have been damaged beyond the point of real repair. :-( lord knows i dated a couple!
there's also plenty of other reasons for antisocial tendencies, such as sensory disorders or even general fatigue, but those are simply unfortunate. many of them ride sidecar with introversion, though! i'm not SUPER extroverted or introverted either way, myself. when i feel the urge to hang out, i try to do so; and when i want to fuck off quietly in a corner and get away from everyone, i do likewise. i just roll with it, because why drape weird interpretations atop chemical responses? (note: i am not opiate and therefore not 100% successful in this attitude.)
but yeah, there's two types of introversion: self-realized, and self-inflicted. we all need people, just not all the time, and some need them with greater frequency than others. it's good to be in touch with when we as individuals do need to be with others, and when we don't, and when we tell ourselves lies of convenience. (nothing worse than an extrovert who needs to TIME THE FUCK OUT, btw.)