Hi oligopoly,
Ideally, the two people in love and in a physical relationship, should also be best friends! The differences are quite subtle: you can talk about things with a best friend that you can't always do with a partner and get an objective, unbiassed opinion or advice. There's loyalty too, and someone there that you can rely on, through thick and thin, without having to see them all the time, or constantly be on the phone. I am blessed with having two such friends, for the past 35 years!
With a partner, things get more personal and there's a very old, but true adage: "you always hurt the one you love". Expectations of a friendship in a "relationship" are always higher and, when not met, often cause discontent and arguments. Best friends tend to take each other at face value, "wharts and all".
Hope this has been helpful!
Barbrajay