Well I think I have a different view from most. I don't think its something specific that is required, like conversation. They come in all shapes and sizes. Some are complex some are simple, its hard to categorize things you need to do.
First and most importantly, relationships are work. They involve sacrifice, flexibility and understanding if they are to survive. That requires time and effort, work of its own kind, whatever the nature of the relationship is; if that requires flowers and a date every week, thats what that relationship requires.
I've watched too many of my friends marriages fall apart because they take the vow as some sort of threshold where they have to stop working. I've seen the same thing happen to couples. People just lost touch with one another, dynamics change in their life, then its over.
I know it sounds simplistic, but it all takes effort, which like any other type of effort, is not easy. If people don't think the payoff is there, then a relationship will suffer.