Why are most songs about relationships?

That is like the equivalent of asking why the caged bird sings? Freedom. In this particular question you posed it would be freedom of expression about feelings by the interaction of two or more people (relationships). When one is entrapped with different feelings --song or any artistic expression-- is what frees the individual. Its like crying on key to a beat or poetry with instruments in the background--moreso a harmonically blended mixture of both. If the heart and/or feeling could speak it would be through music. More importantly music is nothing more than a mere expression on the reflection of life--- life is nothing more than the interactions of ourselves in relation to person, places or things. To get really technical everything is a relationship which is why all these songs can have an endless around of metaphors to explain how it is that one feels.