I've been veering about in my beliefs about romantic love for some time, finding myself by turns either cynical or starstruck with dreamy idealizations, and at last I wonder if I've come upon a notion that explains or supports both positions.
The notion is this: that romantic love is a single fantasy (or even a delusion) that happens to be shared by two people.
The power of fantasy is that it can go far, far beyond reality. So if you have a particular fantasy about what love is supposed to be, and you find someone with a corresponding or at least compatible delusion, the two of you can together achieve something like what is found in all the storybooks, because you're using the same techniques that the storybooks use: imagination, imagery, and the willful disregard for ugly truths that might otherwise intrude.
Love can fail, then, for the same reason that any other fantasy fails us and is discarded. It is difficult to keep a single idea fresh and alive for months or years, to apply the energy to maintain the fantasy in its initial form.
Love can fail also because it takes only one of the pair missing a beat on that maintenance to bring a disjuncture between the lovers' respective fantasies. When the one fantasy becomes two, they will inevitably begin to clash.
Yet if both partners realize that love is a landscape of the imagination, bounded only by the imagination, with two hands upon the brush and two sets of eyes choosing from the palette, then in a collaboration of spirits they are free to render whatever vistas of romance they choose, to range as far and as wide as they can agree upon.
This is probably why monogamy is the ubiquitous ideal of romance -- it is unlikely enough that two imaginations should have the compatibility to sustain a mutual dreamworld over the course of years or decades. What are the odds of finding three able to do so, or more? (Which is a shame, really, since anyone who's been around families with children should be able to quickly realize that as a practical matter, having a third or fourth adult in the house would work wonders for the stress levels and sanity of the parents.)
So then, unless I'm particularly full of it this evening, the trick of it is to find an imagination that can either parallel or mirror your own, and then work very hard, together with that imagination's owner, to paint a place where the two of you can live, and constantly change and update the image that you are imagining in order to keep it vibrant and new.
Alternatively, you could decide that you're not in for that much work.
Thank you, goddess of love, for the notion of romance, and for all those who are able to find it and make it work.