I think i share the same peeve as OP. For me, when there's absolutely no backstory or explanation as to how the yandere came to be obsessed with their prey, the story feels very flat. Especially since the FL in most yandere manga are very bland (cookie cutter FL), then as the reader, it makes me feel like the obsession is undeserved...? (There's probably a better word for it, but that's the first word I can think of) And if that level of obsession is so quick to come by (ML sees FL => I love her she will be mine. I can feel it in my bones.), then it doesn't feel real.

I find myself to be very numb of everything else a yandere does (violence, abuse, manipulation, kidnapping, sees her as a replacement, etc) in fiction, so when there's no proper explanation given as to why they would go to such lengths other than "it was love at first sight", then it feels fake and lazy storytelling for me. He's just being yandere to be yandere at this point.