Relationships are all about emotional synchronization. Humans shoot for quality over quantity when reproducing, and consequently, take a long time to reproduce. It takes a human up to a year to conceive, 9 months for the baby to grow and develop inside of its mother, and up to 12 hours for the mother to give birth to the baby. Human reproduction has a lot of room for error, and since it takes such a long time, can be devastating if it doesn't work out. If a human couple in the wild fails to get it right the first time, they might not even live long enough to have a shot at trying it again. If something happens to the male, female, or baby during the reproduction process, it could very well mean game over.
Since reproduction was such a risky gamble back when we were hunter-gatherers, we developed this neurological electrical-chemical process we call "Love." Love is the 3rd and final stage, in the human attraction process.
The human attraction process consists of 3 different stages:
Lust: Which is accompanied by the release of estrogen and testosterone, is what gives us the desire to find love in the first place. It's the force that drives people to look for potential mates to reproduce with. This stage doesn't begin until adolescence, when sexual organs have matured, and testosterone and estrogen are being released. Since these chemicals are being released in such a copious amount during adolescence, it's common for teenagers to become enamored with other people for superficial reasons. It's also the reason why teenagers, especially younger ones are fixated on searching for mates. It's also the reason why many teenage relationships are centered around sex. It's because lust, and the chemicals associated with it, are dominating the brain. Now before somebody gets upset with me for "Generalizing" teenagers, let me say that every teenager is different. Each teenager has deviations in brain structure, and chemical production. Because of this, there will be some teenagers that act differently than the majority do.
Attraction: Also known as infatuation, this stage is accompanied by the release of dopamine, norepinephrine, serotonin, and nerve growth factor. This is the "Honeymoon" phase most couples go through for the first 3-6 months of their relationship. The two people become enamored with each other, and spend almost every moment with each other. This is primarily due to the heightened presence of dopamine and norepinephrine. Dopamine is the brain's pleasure chemical, and is largely related to happiness and euphoria. Norepinephrine affects hyperactivity, sleeplessness, goal-oriented behavior, and attention. Dopamine and norepinephrine are commonly found to coincide with one and another. This stage of the relationship is there to get the couple to spend time with each other in order to get to know each other. What makes them tick, how they are like, what they desire... What they need. It plays a key role in the couple emotionally synchronizing with each other.
Attachment: "Love." This stage is accompanied by the release of oxytocin, and vasopressin. It is also accompanied by a reduction of the hormones that were released during the attraction stage. Oxytocin also known as the "Bonding hormone," is associated with pair bonding, and separation anxiety. Interestingly enough, it's also released during orgasm. Vasopressin, which amongst several other things, is associated with pair bonding. This is the stage where the emotional synchronization starts to finalize.
Emotional synchronization is essential to a relationship. If the emotional synchronization is disturbed, both members of the couple will subconsciously size up the situation. Depending on what conclusion the members of the couple come to, the relationship will either be repaired, or replaced. Factors that go into this decision include things such as how much as been invested into the relationship, how severe the disturbance is, and the availability of replacement mates. If it's something minor such as someone forgot about the couple's anniversary, the emotional disturbance can typically be rectified by reassurance. But something such as one of the members of the couple moved to another state, and long-distance communication and interaction is severely inhibited over an extended period of time, the emotional synchronization will be very affected over a long period of time, and in many cases, results in the relationship's end. That is the reason why many people are apprehensive about online and long distance relationships: interference with emotional synchronization. Emotional synchronization is also the reason why people often look for themselves in their mates. The more they have in common, the less conflict they will get into, the less disturbance. The more they have in common, the easier it is for them to become attuned to the needs of each other, the easier it becomes to emotionally synchronize with each other.
I don't measure people up to an arbitrary list of criteria. If I like a person, then I like them. What should matter is how I feel about a person, not whether or not they are attractive, male or female, tall or short, smart, or funny. If I enjoy being around a person, then that is all that matters. These arbitrary checklists are irreverent, and only serve to reduce the possibilities and freedom I have to find a person I enjoy being around. They are counter-productive, so I avoid them.