Limerence is an involuntary state of mind which results from a romantic attraction to another person combined with an overwhelming, obsessive need to have one's feelings reciprocated.
In biology, Bateman's principle is the theory that females almost always invest more energy into producing offspring than males invest, and therefore in most species females are a limiting resource over which the other sex will compete. It is named for English geneticist Angus John Bateman (1919–1996).
Homogamy is marriage between individuals who are, in some culturally important way, similar to each other. Homogamy may be based on socio-economic status, class, gender, ethnicity, or religion. It is a form of assortative mating.
The Scarcity Principle
The Ben Franklin effect is a psychological finding: A person who has done someone a favor is more likely to do that person another favor than they would be if they had received a favor from that person. Similarly, one who harms another is more willing to harm them again than the victim is to retaliate.
"all the girls get prettier at closing time" phenomenon
The Science of the Friend Zone