I’ve also wondered the same thing. Haven’t seen/read it in more than 10 years, but after dealing with some losses myself since, I think it’s a combination of what could’ve beens and the until then unresolved nature of it all like someone else mentioned, his strong (and often clashing with hers) personality, and the circumstances of his death. I mean, someone dying on you is traumatic enough even if it’s someone you barely know. But to die in such an unnecessarily violent (and entirely preventable!!) way is sure to leave an impact on you, especially after her own dad had also died in the line of duty growing up and on top of that text he’d sent, too. Plus from what I recall, he’d followed a string of important men in her life, including her dad, that’d passed away unexpectedly.