Well, according to the psychology book, there are a few major roles that pertain to where we find this person. We look at things like proximity, similarity, reciprocity as in if they like you, and then you are more likely to like them back. Then crazy things that some might think are fictional like passionate love. I feel like in some of the TV shows we see today, these things do actually take place and play a major role.
Take the bachelor or the bachelorette as an example. A man or woman is presented with a large group of potential mates that they could end up with and in the end they eliminate people till there is just their one mate left. The role of proximity plays a big role because they are with these people all the time as to get to know them well and try to find the perfect mate, so essentially it has to be one of those people. Then take similarity into account because I'm sure that the people that are chosen to be their potential mate would work well with them because of similarities. And of course take in reciprocity, so if they like you, you're more likely to want them back.