Because setting puts limits on what characters can and can't do it helps shape the storyline. Without these limits a story could easily veer off course and become confusing or unbelievable.

abstinence skills

By saying no you are simply telling your friends that there are limits to certain things. You are not hating them you are simply setting setting the rules.

It places limits on what characters can and can't do.

It affects how the reader will interpret characters' actions.