Giles leaving because Buffy needed to learn how to live without his help was ridiculous, given she'd just returned from the actual dead and clearly needed his help more than ever at that point.

His reasoning should have been that he feels responsible for Buffy's hardships and doesn't want to continue hurting her; that HE is the reason all this happened to her, that HE is the reason she died etc., because he was the one who all but forced her to get back into slaying when she came to Sunnydale, because he trained her and clearly wasn't good enough, and because he represents this centuries old tradition of Watchers and Slayers that ruined a teenage girl's life.

And on top of that, here Xander is getting married, an adult in his own right without need for someone like Giles around, and Willow-- Well, Willow is outright threatening him. The icing on the cake is that Willow of all people no longer respects him, and maybe he is even a little scared of her. And, similar to his guilt over Buffy, he feels responsible for Willow having gotten so into magic in the first place.