A girl wants to feel taken care of by her future husband. When a boy holds open the door for her, she gets this sensation. Of course, he isn’t really taking care of her yet, but her feelings may mean more to her than reality. When the boy fumbles with the keys, it ruins this sensation, and the effect of holding open the door is lost. However, if he doesn’t open the door, she may think that he doesn’t care about her. Therefore, if you forget to open the door, just say “oh, I forgot to open the door.” This way she understands what you were thinking, and won’t hold it against you.
Many boys would rather skip this whole show. It seems pointless to make believe he is taking care of a girl who is really still a stranger to him. You may want to consider this. The first time, open the door and say “it seems a little bit strange to me, but this is what I was told to do.” Many girls will answer “it’s ok, I’ll do it myself.” If she wants it anyway, well, then do so. However, depending on the girl’s personality and her family’s origin, this may be very important to her. She may be offended that he doesn’t show interest in pampering her. Every boy and every girl are different.