What attracts people of the opposite gender to be in a relationship with one another? The answer, for the most part, is similar to what you might expect: the benefits of having friends of the opposite gender.
What kind of role does sex play in the attraction of people in relationships?
I know this is going to come as a huge surprise, but it actually plays a fairly large part. When it comes to attraction of females, friends or significant others, men often choose people that they are sexually attracted to, as they are looking for friends with benefits more often than females.
For women befriending men, this doesn't make as much of a difference, although, what might is whether or not they can provide protection, both physically and socially (Dr. Dylan).
Even more meaningful than sex, however, is the benefits that come from an opposite gender relationship. The most important things people want from a relationship are companionship, conversation, and kindness, all things that might be uniquely experienced in an opposite gender relationship.
It may not surprise you that people who share similar qualities, such as things that people value and personality traits, tend to be more attracted to each other. (Ever heard the saying, "Birds of a feather flock together"?) Implicit egotism, states that -simply put- people like themselves and that people extend that love of themselves to people and things that remind them of themselves. What's not to like about you? You do what you want because you want to, and as a result, if what you do is similar to what other people do, you're more likely to become attracted to them.
Something that might surprise you, however, is that people may actually become more attracted to people with the same initials as their own. (Dr. Dylan) Some psychologists argue that implicit egotism extends that far. It wouldn't be on a conscious level, but the attraction would be happening subliminally. Many psychologists have noted a correlation between the our own and our lover's initials, but, like mostly everything in psychology, there are also those that argue that this claim is lacking enough evidence to be considered true. Either way, it is interesting to note, as there are many examples of celebrities with similar initials, i.e. the recently split couple of Kris and Kim.
*(Make up for emotion discussion)
Dr. Dylan Selterman. "Science of Relationships - - - Sexual Strategies in Cross-Sex Friendships." Science of Relationships - Welcome to ScienceOfRelationships.com. Web. 02 Nov. 2011.