Similarly, when my toddler is melting down because she’s angry at her little brother or she stubbed her toe, she loves when I respond in kind, “OUCH! That had to hurt!” or “AAAGHHH!” or “OH MAN!”
Doing this acknowledges her feelings and lets her know I hear her, I see and understand her, and I’m right there with her. She also finds it pretty silly when Mama exclaims loudly WITH her.