A very interesting essay about Buffy and sacrifice from S1 to S8. A must read.
Becoming throughout the show
Becoming is often seen as one of the key moments in BtVS history. Mostly everyone agrees on it. This is the episode where Buffy had to choose between Angel and the world, and at great personal sacrifice she chose the world, and killed Angel.
And Buffy has been returning to this same choice, the Becoming choice, again and again. Joss has been returning to it. I've seen the repetative nature of Joss and this plot element for Buffy been brought up as a criticism, but for the moment let's look at what it does. There are three central death-sacrifices in Buffy:
1) Becoming. Buffy kills Angel to save the world.
2) The Gift. Buffy is asked to kill Dawn to save the world. Buffy kills herself instead.
3) Chosen. Spike kills himself to save the world.
Becoming throughout the show