As someone who has been with my man for 30 years, 26 of those married, I’m going to confirm the OP’s hypothesis. If you aren’t best friends, you might still love each other 10 or 20 years in, but you aren’t going to have the same kind of quality in your relationship. I know of plenty of couples who love each other but don’t have a very strong connection anymore because they weren’t also good friends. We’ve been through some really hard times together, facing both external circumstances and couples dynamics things. During those times of stress and challenge we turn to each other rather than on each other, because that’s what friends do — they support each other in hard times.