One possible reason could be that the boy misbehaved at school. For example, he might have been bullying other kids, which is unacceptable behavior. His mom spanked him to teach him that such actions are wrong.
It could also be that he used bad language. If he picked up some inappropriate words from somewhere and used them at home, his mom might have spanked him as a form of discipline to teach him proper language use.
He might have cried immediately. Most kids would cry when they get spanked as it's a painful and scary experience for them.
Disobedience is a common one. For example, if a child is told not to do something like play with matches and still does it, the mom might spank as it's a very dangerous behavior.
One reason could be that the boy was being disobedient. For example, if the sister asked him to clean up his toys and he refused, she might spank him out of frustration. Another reason could be if he bullied her in some way, like pulling her hair or taking her things without permission.
In many stories, disrespect is also a cause. Saying rude things to mom or not listening when she's talking. Moms spank to teach kids to respect others, especially family members. It's their way of instilling good values in a rather tough but sometimes necessary way.
Maybe in the story the girl sees it as a form of attention. Sometimes children might misinterpret discipline as a way to get their parents' focused attention. If the mom is often busy, the girl could think that getting spanked, which is a form of interaction, means her mom is paying attention to her.
Perhaps the boy used bad language in front of the friend's mom or the family. In some families, using inappropriate words is considered a serious offense and might lead to such a punishment. Also, if he was causing a general ruckus and disturbing the peace in the household, the friend's mom could have spanked him as a form of immediate discipline.
It's not appropriate. A child has feelings and being spanked in front of others, especially strangers like mom's friends, can make the child feel ashamed and unloved. There are better ways to discipline a child such as time - outs or taking away privileges which can be done privately.
It really varies. In some traditional neighborhoods where people know each other well and have a shared set of values, such stories might be more common. For example, if a boy is constantly bullying other kids in the neighborhood, the moms might step in. But in more modern, individualistic neighborhoods, this kind of direct discipline from a non - parental figure is less likely to occur as people tend to respect the boundaries of parenting more strictly.
One reason could be to show the idea of discipline. In some fictional settings, it might be used to teach the boy a lesson about right and wrong.
One possible reason could be that the boy did something really disrespectful towards the girlfriend or her family. For example, he might have been rude to the mother directly. Another reason could be that he got the girlfriend into some serious trouble, like getting her involved in dangerous or unethical behavior.