The reason the hero is so clingy and acts so forward is probably because of the MC is so distrustful of him due to her past lives.
It makes sense, she's a pretty honest person so I doubt she pretended at all. She was probably the same as she acts now, happy half the time and sad and distant the rest. In the first chapter, the hero asked her to wait for him but she just responded with "Be careful".
He's perfectly aware of her insecurity, that's why he's blatantly rejected the other girls in front of her.