In a herd, all young ones are learning how to run, hide and to hunt; to understand how the jungle works. Some of them make their first kill and the others get weeded out by other predators. In simple words, the fittest survive. The traits that the fit ones carry are selected and continued in the future generations and the traits of those who were weeded out, die out eventually. Thanks to Darwin, to an outsider this seems very natural, but the species going through this selection process has no clue why some survived and some died. It is not possible for them to identify the strong traits from the weak ones.
Darwin’s theory works not just with the evolution of species but with a lot of other things. Let’s just take relationships as an example.
When kids have their first attractions, more often than not while they are in school, they are trying to understand how their emotions work and what they mean; they are trying to understand their attraction for the opposite sex. Most of these early attractions are shallow but a few convert into what we call puppy love. These give way to the early relationships which we start calling love and here the test for the fit and unfit traits starts. People try out all ways to make things work and obviously they are not aware of the traits at that point that will make it successful. Eventually, through natural selection, one relationship survives while all others die out. There are obvious anomalies to this as everything else but this is how it usually works.
Now if we try to understand this as an outsider (which people do in retrospect) it is not tough to observe that the relationships that die out have a lot more insecurities, doubts, trust issues, incompatibilities that are impossible to reconcile and lot more. Whereas those relationships that work can almost always be termed as mature, trustful, compatible; in these, both people have a sense of independence along with a sense of belonging; they feel more free than restricted.