I could not recall seeing the reason being explained in the anime but you have to understand that that people tend to react differently when they are placed in a situation like what happened in SAO. It was mentioned in here that
'when she discovered that she and all of the players were trapped in SAO, she secluded herself inside of her rented room in the Town of Beginnings for two weeks before resolving to take her fate into her own hands.' It was also mentioned as her personality that "she is not afraid to act out on her own and will take matters into her own hands from time to time" and that "Asuna originally did not care about dying in the game before meeting Kirito."
This implies that despite her 'bright, charasmatic, personality,' she is independent. That is just who she is. She seems more of a loner when Kirito first meets her because
she spent most of her time as a Solo Player after her two-week seclusion and because she originally did not care if she dies or not, as mentioned above, so she would most likely feel indifferent toward any group/party she meets. She might be okay with working with temporary parties/groups but at first, she probably do not want to get close to anyone, like being friends, to avoid hurting herself more since anyone could die in the SAO world. A proof of how she values relationships is mentioned in the wiki: '..She also cannot handle seeing others die in front of her, such as when she recklessly attacked The Gleam Eyes after seeing the Aincrad Liberation Force players getting slaughtered..'