It seems a lot of the fandom follows Catty's narrative in thinking Alphys and Mettaton were simply friends and then not-friends, but I've always thought the game's details and subtext indicated something more intense, more passionate and more desperate, and ultimately more toxic.

Ugh. That's a good ugh, an "I didn't realize this angst would feel so right" kind of ugh; a "thought so much about these dorks over the last year I didn't expect hearing yet another playlist would spark new feelings about them but it did" kind of ugh.