I think Buffy was also happy at the end because the pressure was removed from her. She shared the duty with 1800 slayers. She didn't have to hold the burden and responsibilities anymore. She could live a MORE normal life than before, without having to patrol constantly. Right before she smiled, Faith reminded her that what is she gonna do now that she's not the only Chosen One? Faith said Buffy can have a normal life.
TRANSCRIPT:
FAITH
I just want to sleep, yo. For like a week!
DAWN
I guess we all could. If we wanted to.
WILLOW
Yeah, The First is scrunched so… what do you think we
should do, Buffy?
FAITH
Yeah, you’re not the one and only Chosen anymore. Just
gotta live like a person. How’s that feel?
DAWN
Yeah, Buffy. What are we going to do now?
Buffy the Vampire Slayer
Chosen
59
Buffy Anne Summers looks off into the future, a satisfied smile spreading
across her face.
FADE TO BLACK
SERIES END