Daisy had been having such a good day before Leo had a serious reaction to the peanut oil on the fries. Once her panic subsided, she couldn't stop kicking herself for not realizing but shouldn't he have known that? How could he have gone his whole life without eating any peanut products?!
Seeing him get carted off on a gurney made her realize that she wasn't nearly as indifferent to him as she had hoped to be. She had been terrified of losing him.
Not nearly as terrified as he seemed to be though. She had never seen him so freaked out. It made perfect sense once he told her about his previous experience with a fatal allergy. She couldn't even imagine how horrible it would be watching someone you love die right in front of you, especially from something so curable.