In summary, the 80/20 rule as it applies to relationships is that you spend eighty percent of the time doing normal, every day, won't-blow-your-kilt-up stuff, and twenty percent having important, intimate, life-changing experiences. With most relationships you will never get to the sweetest twenty without investing all the basic and normal eighty.
In my own life, I have some really good eighty percent friends. We don't have much pretense, it is about realism, trust, and relaxation. The more real we are, the more we trust, the more we relax. Everyone needs people like that in their life. The people you don't mind seeing you before you've got make-up on; who you never call for bail money, because they're sitting in the cell next to you. In friend terms, these are keepers. You find these in the romance department, just not as often as everyone would like.
The reason this gets interesting for romance is that we have a tendency to sample these experiences in random orders that can leave you drained and devoid of any lasting connection. For example, that drunken hook-up that finally made it clear to you why they call it wild-monkey-sex; the office mate you've been flirting up for weeks and finally find yourself shoulder to shoulder with in the supply closet.
[after surviving the bus explosion]
Annie: You're not going to get mushy on me, are you?
Jack: Maybe. I might.
Annie: I hope not, 'cause you know, relationships that start under intense circumstances, they never last.
Jack: Oh yeah?
Annie: Yeah, I've done extensive study on this.
[the last lines in the movie]
Jack: I have to warn you, I've heard relationships based on intense experiences never work.
Annie: OK. We'll have to base it on sex then.
Jack: Whatever you say, ma'am.
-- from the movie Speed
It is pretty common for us to get a taste of the sweet 20 upfront and assume it will be the norm for the remainder of the relationship. It isn't until we've invested significantly more time that we realize we already milked the best out of it and what we are left with is more like orange juice after toothpaste.
Like a cake, or maybe a fine wine, any relationship will have different flavors as it bakes (or ages). This isn't just normal, it's the point. Only expect a relationship to stay the same when the two parties involved aren't growing either. In this world, it is pretty hard to stop growing. Life has a tendency to drag you kicking and screaming into potty-training, sleeping in a big-boy bed, and pajamas with no feet whether you like it or not. So it helps to not just accommodate the changes, but expect and maybe even embrace it a little. You never know, you just might get a little taste of the sweetness if you take the time to savor.