It may be obvious, but like most wisdom we think we know but which we understand only intellectually, it bears repeating: no two people are the same. We all have different personalities and interests. And even if somehow our personality and interests were identical in all respects to our partner's, it would be even more improbable for us to always react to things identically or want to pursue those interests in exactly the same way at exactly the same time. Thus the need for continual compromise—and from it, sadly, the frequent cause of the dissolution of many relationships.
Or, at least, that's what many of us think. "We just grew apart," we'll say. Or, "We just want different things." And from these sentiments sometimes arise other, more malevolent ones: "I don't love him anymore." "She drives me crazy." "I don't even like him now." Though every situation is different, though relationships are exceedingly complex, and though undoubtedly some couples shouldn't remain together, a more likely explanation for why couples split than one or both partners actually changed (though, of course, that sometimes does happen) is that one or both partners lost their ability to tolerate their incompatibilities. That is, though one or both partners may have begun their relationship with eyes wide open and fully accepting of those incompatibilities (labeling them initially only "differences"), over time tolerance for those differences was gradually lost.
I would argue this happens because of a basic psychological bias: loss aversion. That is, couples split because human beings evolved to notice and weigh more heavily that which causes them pain than that which brings them joy.
Imagine for a moment if we weren't so wired. Imagine if all the good things your partner did, all the good things your partner is, constantly and without any conscious effort on your part powerfully drew themselves to your attention, while you had to continuously struggle to focus on—even to remember—your partner's faults and all the times he or she irritated you or disappointed you. (You might be thinking that the bad actually outweighs the good in your current relationship—and that may actually be true. But perhaps the reason it is true is because you don't naturally focus on your partner's good qualities, nor he or she on yours, so that you are pulling out more of the bad than the good.) If all the work were taken out of appreciation, how would your relationship change?
All experiences—all relationships—are made up of both good and bad parts, of good and bad moments. The way we experience relationships, on the other hand, is a function of what we notice about them, moment by moment. Paying preferential attention to pain may offer us a survival advantage, but it makes relationships hard to sustain over the long haul and explains that oldest of cliches—that relationships require work.
Couples bend and twist themselves into the most uncomfortable positions to rationalize, accommodate, suppress, and ignore unpleasant interactions with one another. But perhaps the reason so many couples fail to stay together in the long run is that these positions eventually become too uncomfortable—that these strategies we use to tolerate our partners run counter to our most basic evolutionary programming.
If we have to work at making relationships work, then, wouldn't we be better off working smart rather than hard? That is, because we can't stop ourselves from focusing on the bad no matter how hard we try, perhaps we should stop trying. Instead, we should make sure to consciously direct our attention to the good, a strategy I discussed at length in a previous post, How To Manage Frustration. That is, we should implement a simple "if-then" rule (meaning, turn it into a habit): every time we find ourselves feeling negatively about our partner for any reason, we should acknowledge the legitimacy of that feeling and then summon to mind something about our partner we like. It doesn't need to be something good in equal magnitude to the bad thing that set us off. Nor—and this is crucial—should we expect it to nullify our negative feelings. Rather, we need only to answer those feelings—to remember that though we're not in charge of what draws our attention, our evolutionary drives can dominate us only if we're lazy.
You may, in the end, decide you and your partner are incompatible anyway. But if so, it won't be because you passively allowed your aversion to pain to paint a picture of your partner's personality and behavior that only tells half the story. That is, it won't be because you didn't make the effort to find enough good to balance the bad. It will be because what good you did find genuinely wasn't enough.