Like attracts like 物以类聚，人以群分 wù yĭ lèi jù, rén yĭ qún fēn Opposites attract 异性相吸 yì xìng xiāng xī
In physics, several theories have made it clear that like charges (or magnetic poles) repel each other, whereas unlike charges attract. When people are involved, however, the laws of attraction are more ambiguous. Some research argues that people tend to marry partners from similar demographic classes (age, education, religion, socio-economic status, etc.), suggesting that Plato’s first law of affinity, i.e., “likes attract”, also holds for relationship life. Other studies, on the contrary, put forward the notion that “opposites attract”, reasoning that people are drawn to individuals whose needs match their own in a reverse way. The object of this section is, therefore, to elaborate on these approaches, and to show that while both may be valid, seeking a balance between “like” and “unlike” arguably promises the most success when it comes to finding a (soul) mate.
The former idiom can be expressed in English in various ways: “like attracts like”, “like begets like”, “that which is like unto itself is drawn”, etc. It was Plato who, building on the conception of philia (attractive force, as opposed to neikos, or repulsive force) originally established the first law of affinity that “likes tend toward likes”, for example, water to water or earth to earth. But also for human beings, it is a rather natural and intuitive reaction to being drawn to people who are similar to oneself, who share similar features, tastes, habits, etc. They instinctively look for the same characteristics in others that they see in themselves. Alikeness creates a sense of comfort and security, which is very important to grow the trust and empathy required for love to happen.
One construct that attempts to explain this observation biologically is assortative mating, under which individuals with similar traits are said to mate more frequently than what would be expected randomly. The advantage of this strategy is that it increases genetic relatedness, which in turn may contribute to improved communication or selflessness between family members. In general, assortative mating occurs across genotypes and phenotypes with similar physiological characteristics (e.g., body size, morphology, bone structure, skin colour, etc.). For humans, however, many other dimensions, such as age, intelligence, socioeconomic status, religious beliefs, political ideology, etc. can play a role as well.
Then, based on the premise that similarity is indeed a crucial determinant of interpersonal attraction, the main question is which aspects and forms of alikeness are required from an individual perspective. Countless studies have addressed this issue. As it turns out, similarity is appreciated for a variety of personal attributes, for instance, family background, appearance, ways of thinking, goals and interests, or leisure activities. Similarity here is regarded as a positive factor that intensifies the initial attraction and eases the development of relationships. Likewise, it has been established that the range of variables affecting the outcome of who falls in love with whom is equally broad. Features include “age, personality traits, appearance, height, weight, eye colour, and other physical characteristics, including physical defects, behavior patterns, professional success, attitudes, opinions, intelligence, cognitive complexity, verbal ability, education, social and economic class, family background, number and sex of siblings, feelings toward the family of origin, the quality of the parents’ marriage, race and ethnic background, religious background, social and political affiliations, acceptance of sex role stereotypes, physical and emotional health, emotional maturity, level of neuroticism, level of differentiation from the family of origin, moodiness, depressive tendencies, tendency to be a ‘lone wolf’ or a ‘social animal,’ tendency to lie and be inconsistent, as well as drinking and smoking habits.” Among all these points playing a role in romantic attraction, three shall now be discussed in further detail: Physical appearance, personality and attitudes.
 Hoffman / Weiner (2003)
 The story behind this saying was brought to us by Han dynasty (206 BC – 220 AD) scholar Liú Xiàng in his compilation Strategies of the Warring States (战国策, zhàn guó cè). The chapter about the Strategies of Qi (齐策, qí cè) mentions an episode in the life of Chúnyú Kūn (淳于髡), a philosopher and official during the Chinese Warring States period (481 – 221 BC). After the ruler of the time, King Wei of Qi (齊威王, Qí Wēi Wáng) had asked him to identify and gather a number of scholars able and willing to serve the country, Chúnyú Kūn immediately came up with a list of seven candidates. The king became sceptical, as he had believed that it would have taken one hundred years to find one smart person alone. Known for his wits and erudition himself, Chúnyú Kūn replied that similar things tend to associate with one another and that people with similar characteristics or interests will often choose to work or spend time together – just like birds of the same species who eat, sleep and fly together. “If I am a solon, a sage and a wise man, all my friends should have a noble character and an extraordinary intellect as well” – thus was the message that Chúnyú Kūn had brought across to his king.
 Pines (2005), pp. 48/49