It's common knowledge that the Shushan Shangqing Faction is a rather netherworldly sect.
Because Sword Immortals heavily rely on the "breakthroughs and sudden enlightenment at the brink of life and death," the sect also encourages peer-level sparring among its members.
The most typical rule is that if someone challenges you to a duel and you refuse, you have to compensate with a substantial amount of Spirit Stones.
Such a culture and environment make everyone extremely combative. Even Senior Sister An, with her gentle nature, had to pretend to be vicious and cruel. Otherwise, she'd be taken advantage of like a soft persimmon.
By that logic, Junior Sister Xu would have been better suited for the Shushan Shangqing Faction.
With her competitive nature, whenever she felt unhappy, she could just challenge a junior brother or sister to a duel, easily defeat them, and have her thoughts cleared in no time.