Well

The dom and sub dynamics are literally explained in the first page of the first chapter. He didn’t like the top bc he appeared as arrogant and rude and literally disregarded him in their first meeting lmao. When they meet again in the rain he gave him a different/better impression and reason as to why he treated him that way in their first meeting (bc his little brother stresses him out lmao) and then he apologized. The sex also wasn’t random the top’s grandma told him he had an arranged marriage and like he said “I wish I could earnestly fall in love with someone” to which the bottom responded “want me to make you fall in love?”. I wouldn’t say they fell in love quickly (maybe the bottom lol) but it took the top some time to ig accept how he was feeling and “give in” to his sub instincts.

I’m ngl u should reread. And a little off topic but I feel like ur taking their sub and dom dynamic for granted. Usually the top is the Dom and the bottom is a sub. It’s so rare for the top to be a sub and the bottom a Dom. Since u said u were confused about the whole dom and sub thing I’m assuming this was ur first dom-verse read. To shortly explain it

Dom
- gives sub commands
-(unfortunately usually the top is the Dom, but not in this bl ofc)

Sub
- Gladly takes/need/want commands (unless it’s a forced command which does happen unfortunately)

Dom and sub are short for dominant and submissive, representing a dynamic where one partner takes the lead and the other willingly gives up control

Ooh girl, this is a bit advanced then (no shade). If you've read a lot of subverse then you'd appreciate how revolutionary this manga is for the genre lmao

This is Subverse, similar to the Omegaverse genre (if you're familiar) where there are secondary biological "traits". In this case, being sub and dom, taking commands and getting commands. Either way, its just another writing instrument to create a shortcut to romance, similar to omegaverses