I think it was because MC was not letting him (the ML) in at all in the start, if u refer back to chapter 38, you can see how the ML is having a hard time with getting the MC to even give him the time of the day, let alone let MC kiss him. I think it also has to do with how MC felt about kisses, about how kissing is something that only people who date do, so knowing this, the ML had to let the MC fall for him since the MC wouldn't let the ML kiss him otherwise.
Yeah, and abt the part of why the ML wanted the MC to hate him, this is my personal interpretation, but the ML, after getting to know MC and realizing that MC is capable to making him "falter from his goals"(again referred in chap 38), he felt guilty for hurting MC like this intentionally and didnt think he deserved MC's love even after all his plans are over and that MC deserved someone better than him, it's the classic trope where "one person does something horrible so that the other person can hate them and move on without any lingering feelings"
But again this my personal interpretation, but please feel free to add your thoughts and interpretations, I'd actually love to hear it since it's also kinda bothering me lol ( ◜‿◝ )♡
Also… why did the ML need to get the MC so totally in love with him. We know the MC only wants to kiss someone he is dating, but why did the ML want the MC’s emotions to get so intense? They were effectively dating ages ago, but the ML just kept pushing and pushing and the intensity of the MC’s feelings just became greater and greater. I don’t understand what the need for this was. He could have kissed him ages ago, but he wanted it to hurt more for the MC because he wanted the MC to really hate him.