You can use the word “relationship” as such. For example: The relationship between diet and health. You can also use words like “connection” or “association”. Examples: 1. Historians study the connection between revolutions and reforms. 2. There is a historical association between war and economic reform.

Do you want to say that "human relationships are the greatest invention in history"? I may have misunderstood your question so maybe you can explain a bit more.

Sorry I don't understand what is your question.