There are theories abound expressing the notion of some kind of "natural selection" process transpiring in the way people choose their mates and lovers.

It's no trade secret that women are only attracted to men who exhibit domineering traits: aggressive, excessively self-assured, smug, arrogant. Even-tempered or milquetoast types they consider undesirable. As are those perceived to be "child-like".
Women want either a surrogate father or a swashbuckler ("bad boy" type).

Men want "trophy" cunts.  They go for the good looks and "sexy" clothes.  If they score a good-looking chick it's like scoring brownie points.  They want to feel like a "real man", and scoring fat chicks or "plain Janes" just won't do.
Everything is visual for the human male.

Now, there are those who contend that this phenomenon is a subconscious preprogrammed instinct put in place by nature for the purpose of ensuring only the most desirable genes get reproduced for the sake of the survival of the species.

However, this notion does nothing to explain why, for example, so many married couples one runs into fit into some of the categories of "undesirables".
Nor does it explain why so many people exist who have profound defects: diabetes; heart disease; epilepsy; mental retardation; mental illnesses; low I.Q.s; arthritis; cancers; obesity; glaucoma; skin diseases; and so forth.

If nature's REALLY "tricking" us into passing only our "best genes" along it's not doing a very good job of it.

(Then again, I never cared much for any kind of Darwinian-minded theories.  They always seem a bit nihilistic---if not literally, then in a manner of speaking.)