Giles and Buffy have a father/daughter relationship, even clueless Travers saw that much. Shipping them makes my stomach churn. It's the same for Willow, Dawn or Xander - Giles's feelings for them are fatherly and familial. Even now, that Giles is a teenager again, I can't see him in any sort of sexual relationship with any of them.
Faith and Wesley never had that sort of relationship. At best, they were co-workers. I can't imagine there is any one, ever, that got along or was friendly with every single person they worked with - they were all just people who worked in the same place as you. It's the same for Wes and Faith, they're never going to be real friends, but they were able to put that aside for the greater good. I will admit, in S4, I could see a certain chemistry between them and I wouldn't have been surprised if that had been explored, had Faith stayed on AtS.
But we still have to remember that Wesley was in his 30's while Faith was, at best, barely out of her teens. At the time, I don't know if the network would have let that fly. It would have been even more controversial for Giles and Buffy - a man in his 40's and a girl, again, barely out of her teens, that he had been an authoritarian role model to since she was 16. It would have caused outrage.
There was also, I seem to remember, a bit of a fuss made when Giles was made a teenager again, when people suggested that he could start a relationship with Buffy or Dawn. Giles still has all of his memories and Buffy is - physically, at least - quite a few years older than he is now. People wouldn't be pleased.
