From the very first season we see this theme, especially in Xander's quest for Buffy's affections. When he finally asks her out and she turns him down, it is the end of the world for him. However, it is coincidentally also actually the end of the world. As Erenberg writes, "There was even a scripted scene (never filmed) in which Xander quietly walked around campus as the sky rained stones on him. A sign of the apocalypse. Just as Buffy turning him down, was a sign of a personal apocalypse for Xander Harris."
The life-and-death nature of love shows up again and again throughout the series. In season 2, Buffy loses her virginity to Angel and he turns evil, breaking her heart. Spike constantly falls prey to the enormity of emotion and pain love can elicit, becoming "love's bitch." Anya and Xander experience this as well, from their engagement on the eve of the apocalypse to Xander leaving Anya at the altar, causing her to become a vengeance demon once again.
But despite the literal apocalypse appearing in these situations, we viewers are always able to relate to the characters. Even Erenberg discusses how Buffy helped him through the experience of his girlfriend cheating on him. Even in the context of vampires, demons, and apocalypses, (as Riley once said, I find myself needing to know the plural of apocalypse...) we can see ourselves in these characters, and in the Scooby Gang's end of the world we can see our own daily apocalypses.