There is no actual reason its just who he is! apart from in seaosn 4 when Willow casts that spell well whatever she says happens and she calls xander a demon magnet
There's no real explanation within the story. After the writers gave Xander 2 demon/villain 爱情 interests (first the mantis teacher and then the Inca mummy girl), it just became a running joke for them and they kept giving him 更多 demon 爱情 interests.