Why do many people reluctant to interact with strangers?

I think the reason why people are reluctant to engage in social interaction with strangers is people are busy.  The more city develops, the busier people become. Many cities have been developed, and the people who live in the cities work thoroughly at their position for themselves or their family. However, these people tend to be reluctant to interact or get involved with strangers, more than people who don't live in the cities. For example, New Yorkers are well-known as cold-hearted people. They don't afford to contact with strangers for any reason because they are already busy enough. People may not think it is worth to act for strangers because it wouldn't be productive. People don't want to make more works for strangers who don't know and don't need to take care of. Busy society makes people non-emotional.