Humans reproduce sexually, so individuals must gain access to a member of the opposite sex in order to have offspring (Trivers, 1972). Human children, however, require considerable, reliable, and prolonged parental investment to have a good chance of reaching sexual maturity. A single parent often cannot meet these demands, making long-term cooperation between parents essential for raising children (Lancaster & Lancaster, 1987). Moreover, pregnancy and nursing substantially reduce a woman’s ability to care for herself and her children, so an investing partner is critical for the survival of both mother and offspring (Hrdy, 2009).

Although creating the conditions necessary to rear children is the primary fitness benefit of long-term mating, it is not the only one. A long-term partner also provides financial and emotional support that can markedly improve an individual’s chances of survival (Apostolou et al., 2023; Buss, 2016). These benefits were especially important in pre-industrial societies, which lacked formal social-protection systems (Boehm, 1999). In addition, because men are on average larger and physically stronger than women, they can offer direct physical protection to their partners and children (Buss, 2016). This protective function was particularly valuable in pre-industrial contexts without civil protection institutions such as the police, where families had to rely on their own resources for safety.

The evolutionary argument outlined above suggests that lasting intimate relationships offer considerable reproductive and survival advantages. From this perspective, human behavior is oriented toward increasing fitness; that is, individuals act in ways that enhance the probability that their genetic material will appear in future generations (Barkow et al., 1992). Individuals, however, do not perform explicit fitness calculations; they do not think, “I need to eat so that I survive and can pass my genes to future generations.” Indeed, such reasoning would have been impossible, because until very recently people were unaware of genes or the process of natural selection. Instead, behavior is guided by emotional mechanisms that implicitly perform fitness calculations and motivate actions that enhance fitness (Cosmides & Tooby, 2000; Tooby & Cosmides, 2008). Put differently, fitness-increasing action requires motivation, and that motivation is generated by emotional mechanisms.

More specifically, negative emotions (feelings that are unpleasant to experience) arise when people encounter situations that jeopardize fitness (Apostolou, 2016; Nesse, 1990, 2019). These emotions motivate corrective action that removes the individual from the adverse state. For example, losing a job can induce sadness, worry, and despair, prompting the search for new employment. Negative emotions also promote preventive behavior intended to avoid recurrence; once re-employed, people try not to lose the new job. Conversely, positive emotions (feelings that are pleasant) emerge in circumstances that promote fitness, motivating individuals to remain in, or to recreate, those circumstances (Nesse, 1990). Receiving a promotion, for example, elicits joy and satisfaction that encourage continued effort toward further advancement.

Because forming lasting intimate relationships confers substantial fitness benefits, strong selection pressures have shaped emotional mechanisms so that they also drive pair bonding. In many cases, these pressures likely co-opted mechanisms that originally served other functions (Gould & Vrba, 1982), such as loneliness and happiness, so that they now promote long-term companionship as well. In preindustrial contexts, isolation from the group entailed serious survival risks from predators or hostile conspecifics, and loneliness therefore evolved to motivate people to seek the company of others (Cacioppo & Patrick, 2008). Selection pressures further tuned this mechanism so that, even when surrounded by others, individuals feel lonely if they lack an intimate partner. Accordingly, people generally experience more negative emotions when single and more positive emotions when partnered. Empirical studies support this argument: Partnered individuals report higher levels of happiness and joy, and lower levels of loneliness and sadness, than do single individuals (Apostolou et al., 2024; Girme et al., 2016). On this basis, we propose that the primary motive for forming lasting intimate relationships is emotional: To experience positive states and to avoid negative ones.

Beyond these general mechanisms, selection forces have also fashioned additional, specialized emotional mechanisms that motivate pair bonding. Romantic love appears to be one such mechanism (Fisher, 2004). When individuals encounter someone who possesses desirable mate qualities, romantic love can arise, binding them to that person (Acevedo et al., 2012; Fisher et al., 2006). Lovers seek frequent contact and miss each other during separation. Because the experience of being in love is pleasant, people strive to form relationships that allow them to feel it, particularly with partners who display desirable traits. Sexual desire represents another relevant mechanism (Toates, 2014). Unmet sexual desire feels aversive, whereas sexual gratification feels pleasant; consequently, sexual desire motivates the pursuit of intercourse. A long-term intimate partner provides a reliable outlet, so people are motivated to establish such partnerships in order to satisfy this drive. Like romantic love, sexual desire is sensitive to partner characteristics; individuals find those with desirable traits more sexually attractive and therefore pursue relationships with them (see also Buss & Schmitt, 1993, Li et al., 2002). In sum, we hypothesize that lasting intimate relationships are sought primarily for emotional reasons: to attain positive feelings, to avoid negative ones, to remain close to the person one loves, and to satisfy sexual desire. Current evidence offers preliminary support for this hypothesis.

It can be argued that people form intimate relationships to obtain the benefits these partnerships provide. Accordingly, what individuals consider beneficial in a relationship can reveal their motives for initiating one and, in turn, may correlate with the motives for maintaining it. In a mixed-methods study with a Greek-speaking sample, Apostolou (2022) identified 58 reasons for staying in an intimate relationship, which were then grouped into nine broader categories. The highest-rated reasons were having a supportive, compatible partner who shares similar goals and with whom respondents enjoy satisfying sex and a strong emotional bond.

A subsequent study examined directly which benefits people associate with intimate relationships. Using a mixed-methods design and another Greek-speaking sample, Apostolou et al. (2023) identified 82 perceived benefits and organized them into ten broader categories. The top-rated benefits were “Love and passion,” “Positive emotions,” “Support,” “Having someone to keep me company and to do things with,” and “Sexual satisfaction.” These ten categories were later grouped into two higher-order domains: “Intrinsic benefits” (e.g., love and passion) and “Extrinsic benefits” (e.g., social acceptance).

To our knowledge, only one study has examined directly the motivations for forming an intimate relationship. McDonald et al. (2025) developed a 24-item Relationship Pursuit Motivation Scale by adapting an instrument created by Sheldon et al. (2017) to assess six motivational orientations. The highest-rated orientation was “Intrinsic” (e.g., “Because I enjoy being in a relationship,” “Because I feel good being in a relationship”), followed by “Identified” (e.g., “Because I strongly value being in a relationship,” “Because being in a relationship is personally important to me”). Third in the hierarchy was “Positive introjected” (e.g., “Because I would feel proud of being in a relationship,” “Because I want to prove to myself that I am capable of being in a relationship”), followed by “Amotivation” (e.g., “There is no reason I want to be in a relationship,” “There is nothing that really motivates me to be in a relationship”). The remaining orientations were “Negative introjected” (e.g., “Because not being in a relationship would make me feel like a bit of a loser,” “Because not being in a relationship would make me feel as though something is wrong with me”) and “External” (e.g., “Because it would make the people close to me happy if I were in a relationship,” “Because it would be easier to maintain my social connections if I were in a relationship”).

Apostolou (2022) explored why people maintain intimate relationships, whereas Apostolou et al. (2023) examined the benefits people associate with forming one. Although both studies illuminate aspects of motivation, they address different questions and therefore do not fully explain why individuals pursue lasting partnerships. McDonald et al. (2025) attempted to tackle the issue directly, but their analysis relied on an instrument designed for general motivation, raising concerns about validity. Because the motives for initiating an intimate relationship are both complex and specific, a scale developed from the ground up is required—an aim the present work seeks to meet.

Study 1 used open-ended questionnaires to identify the full range of reasons that lead individuals to form an intimate relationship. Study 2 employed quantitative methods to group these reasons into factors and higher-order domains. We expected these structures to be dominated by emotional motives; nevertheless, given the limited research to date, we treated the investigation as exploratory. Study 2 also assessed the relative importance of each reason and examined sex and age effects, without making directional predictions.