Because they haven't interacted after S7 at all. They haven't shared a scene together. Xander is mostly paired with Buffy and Dawn, Willow with everybody who isn't Xander. They don't come off as best friends anymore. Best friends spend time together, check on each other, talk with each other, that's just not Xander and Willow anymore.
Not Xander and Willow. The writers seem to take their friendship for granted. It happened in the last seasons of the TV show (which is why the yellow crayon speech doesn't really work for me) but back then they at least had scenes together and occasionally even had (gasp!) one on one conversations. Nowadays they avoid each other as the plague. The utter lack of any Xander/Willow interaction for me is one of the biggest flaws of the comics and god knows I can list a
lot of such flaws.
Slight spoilers for 9.09:
My problem with the PTSD story line can be summarised with "What, now?". Come on, you can't just have a character kill his best friend in the second episode of the show, pull him though hell and back over the course of eight seasons with little to no signs that he is deeply traumatised by the horrors he has experienced and then have him suffer PTSD after a long time-skip. I don't know, it just seems a bit contrived.
Not to mention they moved on from Angel's "catatonia" so quickly one barely managed to say whitewashing before he was back, all dark, mysterious and heroic. I don't think the writers are up to the task of writing such a serious condition. Just look at Buffy's arc this season - it's incredibly simplistic and repetitive (albeit full with bizarre twists). If Xander does have PTSD, I reckon he will be magically cured soon.
For my money it could be simple frustration - vampires and demons are public knowledge yet once again Buffy and her pals need to do the dirty work. I suspect Xander still has an itch to help people but was hoping he wouldn't really have to do it any more. And now zompires are killing police officers as if it's Sunnydale all over again and idiots like Severin are actually lining up to be vamped. Maybe I am projecting but I think Xander must be going all "Is it never over?" Him helping Dowling can also be his way of hoping somebody in charge gets a clue. They just need to stop the guessing game and tell us what's his problem soon.
As for his protectiveness of Dawn, when I read that, it bugged me at first but after remembering that Xander has lost Anya, Buffy (for a few months), Renee, Tara, Giles, etc, the possibility of losing Dawn should always be in the back of his mind.
