Why are people reluctant to engage in social interaction with strangers?

I think people's lives are very busy, and they are often too rush, so people are reluctant to engage in social interaction with strangers. Nowaday, people pay attention only their work and lives, so this enviorment of a life make people are self-absorbed and uninterested to other people. When they walk on a road and use transports, they can meet many stranger who want to get a help such as a homeless. However, people want to avoid the strangers because this situation that people interact with stragers makes people too tired. Also, they want to focus their lives and emotion. In addition, people don't want to make problems. Especially, the news report that helper gets a trouble because of helped strangers. Therefore, people think if they help strangers, they may get caught into a trouble, and their works and time get nagetive effects. For example, some people want to get additional help to helper, so a helper want to avoid these situations. We have to focus a fact that we can't distinguish if the strangers are reliable people or not. Therfore, people think that they have to be careful the strangers. Also, people shift a responsibility on to another's shoulders. They are busy, so they think that someone may help strangers. Strangers are not people's family or friends, so they know that they don't need to help strangers because helping the strangers doesn't bring advantage.