Despite harboring similar 'am I a terrible person for being so happy to see Dawn here?' feelings to Buffy, Xander has pretty much decided to ignore them. Because it's Dawn! And they're not in any immediate danger, therefore being happy is allowed-- and clinging is encouraged.

"It's good to see you. And all in one non-horsey-giant-doll-shaped piece, I'm so very pleased to note." He observes cheerfully, before throwing Buffy a smile, belatedly. Dawn-hugging requires full attention.

"Hey Buff. I interrupted the explaining, didn't I?"