I think it's because the progression is faster than usually is in another mangas
It also actually kind of helps to know that the MC acknowledges right at the outset that this is his future wife. So you know that there's no chance of BS manufactured drama breakups, or harem idiocy, or anything like that. It's just two people discovering their feelings and having a reasonably adult relationship, albeit one where the MC is nerdy enough to send her a scheduling spreadsheet to plan their dating availability, which is
FANTASTIC. As someone who is a bit of an Excel nerd I don't think that even I could ever go as far as to give somebody a spreadsheet with pie charts and schedule tables as part of a romantic gesture. 😂
For obvious reasons this feels like a spiritual sequel to Sweat and Soap in that it's romance with characters that behave like adults instead of all the usual cliche rom-com manga stupidity that authors use to drag their stories out because they think a relationship resolution is the kiss of death.
EDIT: It also helps that Natsu's design is like right up my alley. I know lots of people are probably looking at the coy smile on page 8, but for me that tilted-head smirk on page 6 is just heart-meltingly good.