That “but” poisons the whole phrase, because it neatly proves that whoever is saying it? They don’t love you at all. They might like you, but they don’t love you. They might love what you can do for them (or perhaps to them), but they don’t love you. That’s because love is uncontrollable; it is accepting of flaws, because it has no choice. Love is tolerant of mistakes, because it can’t do otherwise. To love someone, truly love them, is to be unable to let anything else in the way.
Maybe they did love you once, but the moment you hear that “but…” it’s a sign: it’s over. Whatever feelings they had for you are gone, and now they’re using your feelings to their advantage.
Because that “but” always leads to a request. Cut your hair. Change your clothes. Stop hanging out with your friends.
Change yourself into what I want you to be.
Because I’m the one who’s important.
-- Why Does He Stay With Her? by YorkNecromancer
The linked article is actually about the Ruinous Powers of Chaos in the Warhammer 40,000 setting. ^^; But as you can see, the author is pretty committed to making them work narratively ... and in the process, wrote the most frightening depiction of them that I've ever seen. Because the author put into words very well what causes abuse, and what the nature of evil is.
Content note: A couple of questionable (but non-explicit) visual metaphors that could be considered sexual harassment, detailed descriptions of intimate partner abuse, and dudebro(s) being dudebro(s) down in the comments.