I don't know if I would agree that S5 is "stable." From an outsider's perspective, the group's dynamics are certainly more stable than in S6, but there was a lot of upheaval in S5, too (Riley left, and I think we're supposed to assume he and Xander were friends; Joyce died, and that affected all the Scoobies; they learned one of their family wasn't even real; Glory had them more scared than any previous villain), and given how everyone tries to put on a good face in S6, I'm not sure someone living it would realize/react to, 'oh, this year is so much worse than last year!'
Arguably Giles leaving had a big impact on Xander, but I never got the impression that Giles much cared for/liked Xander in the first place and was a father figure to him. Also arguably, maybe the fact that they don't think they're in a lot of danger (because they don't really see the Trio as a threat) for the first time in years makes Xander regress because for the first time he's faced with the prospect of actually living a long-ish, healthy life and facing the consequences of all his choices.
There are certainly arguments for both sides, but for me, personally, I saw his regression as more contrived than natural, and I thought it was a cheap cop-out on the writers' part.
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Arguably Giles leaving had a big impact on Xander, but I never got the impression that Giles much cared for/liked Xander in the first place and was a father figure to him. Also arguably, maybe the fact that they don't think they're in a lot of danger (because they don't really see the Trio as a threat) for the first time in years makes Xander regress because for the first time he's faced with the prospect of actually living a long-ish, healthy life and facing the consequences of all his choices.
There are certainly arguments for both sides, but for me, personally, I saw his regression as more contrived than natural, and I thought it was a cheap cop-out on the writers' part.