Answer:
Because everything you do and say comes full circle, meaning it may happen to you. God says to love thy neighbor as yourself. In other words, treat people how you would want to be treated. He also says that He will bless those who bless you and curse those who curse you. If your friends try and curse someone else, they are really cursing themselves because the man who starts digging another's grave ends up falling in it. Hopefully after reading this, your friends are inspired to make better decisions.
Have a blessed day!