Regularly saying hi to others in the neighborhood was not only linked to improved social success, but also to better financial, career, physical, and community health. 
How so? Being neighborly may increase the likelihood of being outside and moving the body (and vice versa), can foster a sense of belonging and community, and indicates feeling safe when getting out and about.

The survey from Gallop found that adults experience an increase in overall wellbeing when they regularly say hello to more neighbors, with "the sweet spot" being six greetings.

The survey was conducted earlier this year and involved more than 4,500 U.S. adults.

Source: Gallup

https://news.gallup.com/poll/509543/saying-hello-linked-higher-wellbeing-limits.aspx