They are all so happy. She should be happy for them.

Xander and Anya are getting married! (But she isn't sure if it is really going to last.)

Willow and Tara seem very happy together too. (Something seems off about that, but she is too caught up in herself to care.)

Giles is kinda ready to leave, but he's ecstatic that she's back. (Except he keeps on glaring at Willow, like he's pissed at her for doing it. Buffy's tired. She's sure it's nothing.)

Spike is…Spike. (He's never really happy, anyway.)

But she doesn't like happy endings because hers wasn't.