The question of whether or not Buffy jumping to her death in Season 5's The Gift should be read as a suicide attempt has been a somewhat contentious one for over two decades. But whatever you think about the scene or the episode itself, I think it's hard to argue against the idea that Season 6 wants us to read it -- retroactively or otherwise -- as a(n ultimately unsuccessful) suicide attempt, and moreover one motivated by factors other than (though of course including) a desire on Buffy's part to protect her sister.
Other people before me have pointed out that Buffy's first words to Dawn in The Gift after making the decision to jump in her place ("Dawnie, I have to") are echoed twice in Season 6: first when a newly resurrected Buffy makes a literal attempt to jump to her death again (in Bargaining) and then a dozen episodes later (in Dead Things) when Buffy (believing she was the one to kill Katrina) wakes Dawn up before leaving to turn herself in to the police (a metaphorical suicide attempt, paralleling what Faith does in Season 1 of Angel). But something I only noticed recently -- and that I don't think I've seen pointed out before -- is that these two moments are actually the only time in the first thirteen episodes of the season that Buffy ever calls her sister "Dawnie".
Willow uses this form of her name a lot during this run of episodes (twice in Bargaining, eleven times in Wrecked, once more in Gone). Xander uses it once (in After Life). And Tara uses it (in Flooded, then again in Wrecked).
But Buffy? Outside of those two echoed lines "Dawnie, I have to"? To Buffy her sister is only ever "Dawn" until Older And Far Away, the very first episode after Dead Things. And then ... well, just once more, in fact, before the end of the show: in Season 7's Lessons. Willow and Xander both continue to call Dawn "Dawnie" in Season 7 (and Spike refers to her by that name once too, for the first and only time, though not in her hearing). But Buffy doesn't, that one time in the season opener aside.
Buffy doesn't call Dawn "Dawnie" very much in Season 5 either: that's something that Wllow and Tara and (of all people) Glory tend to do. The first time she uses this version of her sister's name is in Forever, in their scene alone together after Dawn has tried to bring back Joyce, when Buffy is trying to explain why she feels she has to take on the role of Dawn's mother now ("who's gonna take care of us?"). And in fact that's also the only time, before that first "Dawnie, I have to" in The Gift.
In total Buffy calls her sister "Dawnie" on screen just six times: fully half of those times are followed by the words "I have to". Without that phrase she'd use the name just once per season. Which, to me, makes those repetitions feel even more deliberate. Saying "I have to" in isolation could just be coincidence: those are all pretty common words, after all. But calling Dawn "Dawnie" first makes it hard to argue for a mere coincidence. That just isn't a name Buffy calls her sister by unless things are pretty desperate.
