Two examples can be used to illustrate this idea:
Firstly, if you think about it, how would a stranger be able to identify the different types of relationship that you have in your life?
How, for example, would this stranger know which person was your father or mother, brother or sister, employer or lover? The simple answer is that, simply by looking at you and them they would not know. They will only be able to guess at these relationships by the way both you and these other people behave.
Secondly, think about what would happen if you claim that a relationship exists and people around you deny that it does.
What would happen, for example, if you went up to someone and started behaving towards them as if they were your boy / girlfriend and they asked you what the hell you thought you were doing?
One answer is that one of you would run the risk of being classified as "mad" - an extreme social category (label) we use to help us explain behaviour we do not understand.
As should be apparent, therefore, the meaning we give to the relationships we identify is very significant in relation to how we behave (and expect to be behaved towards) in particular social situations.