Okay, unpopular opinion.
In order to make our MC's romantic relationship feels "natural" and not "insta-love-y", it's better to make both characters "deserve" each other. There should be a "reason" for character A to be attracted to character B; something intrinsic, yet related to each other's existential "need".
Say, character B is afraid of dogs and always feels like he has to depend on himself to survive (can't even tell his family that he's bullied at school). Make character B falls for character A when she "saves" him. For example, when B is paralyzed with fear in front of a dog, A sweeps in to grab the dog and take it away, oblivious to the fact that she just unintentionally made a guy fall in love with her.
Now, say character A is someone with trust issue. Makes her act nice but distanced from everyone and never believe a single compliment thrown at her. Now, character B who's already fallen for character A sticks to her no matter the circumstances, slowly gaining A's trust and makes her eventually fall for him.
WOULDN'T THAT BE MORE SATISFYING??!!
(yes, I'm talking about you 'A Good Day to be A Dog' webtoon/KDrama :p)