Why?
Because Booth always told Brennan not to look happy at a crime scene lol

DAVID: We all survived. But yeah, I just remembered that.
EMILY: How did you not remember that before? That sounds traumatizing.
DAVID: Because… at the time I got stuck- I didn’t hit my head-
EMILY: You were drunk, that’s- Be honest.
DAVID: Are you kiddin’ me? I was outta my mind.
“he’s my guy.”