Xander, Willow, and Jesse meet, make friends with, and are kept away from the darker sides of Sunnydale by friendly demons starting when they're fairly young, because none of them want to spend time at their homes and their parents don't want them at all, much less want them there. They have no idea there are evil demons, or at least demons who are more evil than humans- they see demons as just people, albeit strange-looking people. And then Buffy comes to town and Jesse gets vamped and staked.