Before we delve into this topic deeper, I advise you first read the post on positive and negative sexual selection. Essentially, both these types of sexual selection work together, and it must be stressed that sexual selection of whichever type is a primal drive of life, equal to the other drives typically seen as primal, such as feeding, survival, etc.
It might seem that examples of modern life sexual selection in humans, such as a mere facebook "like", or supporting Chelsea FC, are not big enough or relevant enough to be caused by a drive as essential and primal as the sex drive. But remember, this drive is of utmost importance in evolution, because it shapes humans' best bets when it comes to unforeseen selection pressures. A spirit of competition and fair-play, for example, might well play a key role if the future selection pressure involves people trusting each other to work together.
How does all of this relate to relationships between people? Relationships may be seen as alliances between people, things or ideas. So naturally, they tend to form as a result of these alliances. Of course, we can all guess which things go better together, that a couch potato is unlikely to marry a gold medallist.
Positive sexual selection is the initial drive behind selecting partners. Agreeing on similar things, doing similar things, and supporting the same attitude, or best bet for the next selection pressure. This could be a political view, a certain activism, or even the seemingly counter-intuitive "who cares" attitude. Sometimes, negative sexual selection brings people together. This can be seen in those who love each other because they hate the same things. To them it is not about focusing on promoting certain things, but about destroying others, be it religion, socialism, or a certain lifestyle.
We have many relationships in life, friends, acquaintances, family. The depth of these relationships comes from the amount of common attitudes towards things we pick to select. Acquaintances are people we have a few common things with. How about that acquaintance that you liked for decades, then found out something about them, and instantly started disliking them? That is long-distance negative sexual selection, where you decide to be against that acquaintance. When it comes to primary partners, things get complicated because you don't only have to consider that single thing that made you dislike your acquaintance. Usually, there are a lot of different things to weigh out. This is why sometimes friends are against your partner, because they have a few items of information about them that they disagree with, while you know a lot more, and find it harder to decide.
Relationships break when these small things add up to too much stuff one partner cannot longer support, be it a change in lifestyle, unforeseen events which change circumstances, or even not enough excitement. On the other hand, it may be just one thing that one partner deems too much to keep the partnership going. A perfect example is cheating. Evolutionarily speaking, it is crucial for sexual selection to work properly and not be compromised. This is why we are so fierce about, and ultimately our whole life boils down to, the things we are for, and the things we are against.
Read the next post to find out why evolutionarily speaking, cheating between partners has become so unacceptable, yet at the same time many people do forgive it. No, don't listen to the psychology blabber, it has nothing to do with babies. Not nowadays, at least, and not from the point of view of evolution.