The bible gives this answer at Pro 13:20 -
"The one walking with the wise will become wise, But the one who has dealings with the stupid will fare badly."
How can/does the presence of others have an hardwearing effect? Influencing a difference in our behaviour?
It has to do with social acceptance. Whatever social circle you find yourself in the most, MAY have an influence on what you consider socially acceptable behavior. Call it an ingrained "survival Self" trait that most pack animals (Including humans) have.
Now, based on your Experiences and Environment growing up . . . we tend to move TOWARDS those social circles we are more comfortable (Understand) in. If we are truly honest with ourselves and are able to look at our actions objectively, we cannot help but know this to be true.