Which is EXACTLY why the monks made the Key into Dawn, she was the little sister, the little girl, the precious thing which the world loves beyond all else, the person whom everyone would gladly give their lives to save as indeed Buffy does.
Everyone would gladly give their lives? Exaggeration much?
Not to mention that the Monks idea is for Buffy to
protect Dawn, not kill herself for Dawn. The monks didn't seem to have the slightest idea that Buffy magically became a spare key as a result of Dawn's creation (probably because that didn't make a lick of sense but let's not digress). In most other cases it would be much better for Dawn if Buffy ran away from Glory, instead of sacrificing herself for Dawn, only so that Glory could capture Dawn a second later.
Plus, I find the idea that Buffy would protect the Key with her life only if the Key is in the form of her (not really) little sister incredibly offensive for Buffy. By protecting the Key she is protecting the lives of everyone on Earth. If the only way for her to fathom this is by putting a bunch of fake memories into her head...then Buffy isn't the same girl who went to fight the Master, staked Ford, sent Angel to hell and is always putting her life on the line for the greater good. In fact, by telling her friends "if the ritual starts we all die" in The Gift Buffy betrays her sacred Calling big time, not to mention her friends.
For me having a hard life doesn't justify acting like a brat.
It doesn't justify it but it makes it more understandable. Personally, it really bugs me that the tough life excuse is very often applied only to Buffy. I have lost count of the times I have seen her actions in Dead Man's Party, Revelations, Empty Places, The Gift, etc excused with the words "she is depressed", "she is under a lot of pressure" and so on. And don't get me wrong, I often agree with such interpretations. Buffy is incredibly tough mentally but everyone has a breaking point. I just think it's unfair when the same justifications are rarely applied to other characters. Just because Buffy's burden is generally bigger than Dawn's or Giles's or Willow's doesn't mean we should dismiss their problems. Dawn's mistakes never led to anyone's death which is more than I can say for Buffy, Willow, Xander, Giles, Spike, Angel, etc.