You should do favors for others.
Sandra's sister asks her to go to the store to buy her something as she is too tired.
Sandra wants to respond to her sister.
Sandra says she will go to the store for her sister and asks her sister to help her too the next time she asks.
Sandra says she won't go to the store just because her sister is too lazy to go herself.
Sandra's sister is grateful for her help and is happy to return the gesture in the future.
Sandra's sister is annoyed that she won't help her and decides not to help Sandra next time she asks for help.