why is it that xander is always attracted to demons?

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

Xander isn't always attracted to Demon's. He and Cordie was a couple. But yes he seems to be getting together with a lot of demons!!!!

There's no real explanation within the story. After the writers gave Xander 2 demon/villain Cinta interests (first the mantis teacher and then the Inca mummy girl), it just became a running joke for them and they kept giving him lebih demon Cinta interests.