You and giyuu were just going to his friend's party to show that you and him are official now, you remained silent the whole journey to not bother him as your very shy

when the car stops at a red light which looked like it will take long, you felt thirsty due to the heat outside, so you reach for the water bottle but before you could say it he gives you some tissue. You get confused but still you used it to wipe your sweat then you reach again for the bottle but he gives you his phone. You become more confused and look through his phone, surprisingly it's your birthday as his pass code. Feeling a bit happy but still thirsty you for the last time reach out for the bottle but he gives you a stack of money instead. It seems like he is misunderstanding your intentions.