It's definitely a matter of perspective. To Kat, losing memories is losing everything, but from Carlisle's perspective... well. Kat doesn't know any of that, and it's probably for the best right now. She might tell Carlisle about "YOLO" and honestly that mind not go so well. She's better about handing out YOLO as advice these days, but still.

Okay he walked right into this, so: "Not really. How tall are you, anyway?"

Humor her, she'll get serious again in time. ...maybe.