It's rude to startle others.
While John's sister Mary watches TV in the living room, John is in the kitchen struggling to make enchiladas.
John wants his sister's help making dinner.
John calmly gets Mary's attention and kindly asks for her help in the kitchen.
John sneaks up behind his sister and grabs her shoulder to get her attention and ask her for help.
Mary agrees to help John and the enchiladas turn out great.
Mary is startled and irritated that John scared her, so she declines to help him cook.