This may be because human bodies are genetically made to reproduce.
I like this point and connection you made on how the reason why relationships rule our lives is because our biological purpose and function is to reproduce. But not only do we reproduce, we do several other things that make relationships messy, as you said. Not only does the scene with the Grandparents mean multiple things are bound to occur but this moves the book forward and keeps us guessing on what is going to happen next. Asking the question, does love outweigh the outfall of relations we have with people and I think it all depends on the person you ask and how important those relationships are to that specific person.