To create genetically unique offspring an organisms must share 1 of each of its chromosome pairs with another's

If a chromosome is n, then a pair of chromosomes is 2n

So, the organism has 2 pairs of each chromosome = 2n

Then it halves its number of chromosomes = 1n

It does this by giving one of each pair to its sex cell = 1n

Two sex cells meet (egg and sperm), then 1n + 1n = 2n 

This 2n cell is called  a zygote

This zygote will grow into a new organism

It has 2 alleles for each gene

e.g. gene for eye colour has two options now: dads blue, mums green

A lion looks at lioness and thinks, I think our combination of alleles will create strong offspring....

Well kind of. You think this too, but you don't realize it.

When you look at a person who is 'fit' or attractive, you are subconsciously weighing up their genetic fitness. 

Genetic fitness is the likelihood of your offspring surviving to have their own offspring - to say it another way, your evolutionary success is determined by having grandchildren

So in males, physical fitness, big muscles indicates the ability to hunt and get food for the offspring, and that the offspring themselves will have those traits and will be successful at hunting, gathering food and fighting off waring tribes

In females, physical fitness comes down to child raising abilities - such as the wide hips and strong legs for moving camp every rainy season

If you find someone that meets the genetic fitness requirements, the cost of offspring is different

Females grow the embryo, fetus, neonate and nurse the infant 

Females will take the main role in the raising of the young

Males risk far less

Because of this, mate selection is primarily driven by females - they pick.

• Researchers have found that upper body muscle mass is the biggest factor for making a male attractive to a female - your pecs, shoulders, biceps and triceps 

Because females choose, most dating patterns in human society are based around this - for instance, to have the best luck, a males ask females out on a date. As its the females that say yes or no. 

The bonding of Male and Female for a lifelong relationship - monogamy - is an evolutionary trait that increases the chances of the children reaching adulthood and having their own children. This means that the emotions of humans is geared towards monogamy. 

In times of low male numbers, especially early in our species history, it was common for a male to have several female life long partners concurrently. This one male will be genetically invested in all of his offspring and thus the well being of the offspring of these women. This also tends to increase the female drive towards monogamy but the male tendency for polygamy - to move his alleles around multiple partners to increase his genetic success

This does not work the other way round. If a female has multiple male partners, then they don't have genetic investment in the child as the father is indeterminable. Thus they are not invested in the offspring's well being and will generally not help. This is referring to ancient times. It is worth noting that step-fathers and adopted fathers will invest in their non-genetic offspring especially if it will enhance the success of their genetic offspring due to providing a happy environment.

So, it 'costs' to have offspring. The cost is greater for females so they subconsciously do the mate selection. But it will still cost both parties.

So why bother?

Well, for most it starts with an urge.

An urge is a neurochemical impulse to do something 

An urge to eat food for instance

What about the urge to have sex?

Well it uses many of the same brain pathways as the urge to have food

Unfortunately, it also uses the same pathways as drugs, so keep away from those cause they will seriously do you damage in the long run

The urge starts with seeing someone who is genetically fit

This allows your brain to get two things 

1. Testosterone

2. Dopamine

The more testosterone there is, the more dopamine there will be - so, usually if there is someone fit, a person may stare at them, thus increasing their levels of testosterone and dopamine. Dopamine makes you feel happy

But, women and testosterone? - Yes, your adrenal glands (that sit on top of your kidneys) release testosterone - this drives the female interest in sex

For males, they already have a lot of free floating testosterone

Testosterone is a steroid, so it makes muscles big - this makes it easier for males to put on muscle than it is for females

This testosterone also means that males will seek out genetically fit individuals to look at

Anyway, then there is dating and eventually the sex

During the sex, the brain is flooded with more feel good dopamine and adrenaline. At the same time the frontal lobe gets quiet so you stop thinking about other things.

At the end of the sex the brains of both people is flooded even more dopamine, but also prolactin that counteracts all that adrenaline and makes you feel relaxed and possibly sleepy.

 It releases another feel good hormone, serotonin. Serotonin makes you feel happy overall - like you have achieved something. Finally, it also releases a third feel good hormone, Oxytocin. Oxytocin makes you feel closer to people.