Friend zone refers to a situation where there is a mismatch of romantic feelings between two individuals. The key to any good relationships is the presence of mutually-satisfying social exchange. In the case of friend zone, as pointed by J eremy Nicholson in his Psychology Today article , some have entered into an exchange that is not fair or equal. "The other person is getting everything he/she wants...but the person stuck in the friend zone is not fully satisfied. In a nutshell, the friend zoned person sold himself or herself short.

1. Bateman's principle 

Bateman's principle is a concept in evolutionary biology which states that males are capable of producing millions of sperm cells with little effort, while females invest much higher levels of energy in order to nurture a relatively small number of eggs, due to which the female plays a significantly larger role in their offspring's reproductive success. Bateman's paradigm thus views females as the limiting factor of parental investment, over which males will compete in order to copulate successfully. 

Essentially, what Bateman pointed out was that women are very picky compared to men in choosing a mating partner because they invest much higher resources and energy in reproduction compared to men. 

2. Assortative mating 

Now we have established the fact that it is females who make the call when it comes to picking a romantic partner. We should now look into the factors that influence one's decision in picking a mating or romantic partner. This is explained by something called assortative mating. 

Assortative mating is a mating pattern and a form of sexual selection in which individuals with similar phenotypes mate with one another more frequently than would be expected under a random mating pattern. In humans, assortative mating is based on genetic factors, socioeconomic status, racial or ethnic background and religious beliefs. 

3. Matching Hypothesis 

The Matching Hypothesis developed by Elaine Hatfield in 1966 says that individuals who end up romantically linked to one another over time tend to match in their general level of desirable characteristics. Individuals who end up in good relationships often match each other in traits such as physical attractiveness, or education, or social status. Although there are exceptions, people tend to attract and mate with others who are similar to themselves, concludes the hypothesis. 

Why does one get friend zoned? 

One of the prominent reasons why people end up in the friend zone is that they are too afraid or passive to express their romantic interest. Many people approach someone they are attracted to as a friend since that carries less emotional risk. Essentially they are using friendship as a back channel for establishing a romantic relationship.