I chose to look at the guidelines for forming relationships; and specifically, in the famous novels written by J.K.Rowling: Harry Potter.
Of course everyone now knows that Ron Weasley and Hermione Granger end up together, it didn't come as a surprise to many avid Harry Potter fans, but especially when re-reading (don't deny you haven't) the books in the middle of the series, one might pick up on more clues that even lessen the surprise.
First of all, proximity is a given. They went to the same school, were in the same grade and even house. Gryffindor represent! Also, they end up becoming best friends and who doesn't basically spend all their free-time with their best friend? Not to mention they got to watch each other grow up and mature in to much more attractive versions of themselves.
The second rule is similarity. They have their differences, but also share some similarities. A main one is magic, they both go to a school of witchcraft and wizardry which is slightly out of the norm for most people. Another similarity is how they both love to hate one another, even though they are great friends one can't help but notice how much they fight/flirt with each throughout the books. A third similarity is how they both get to play supporting roles to the one and only Harry Potter. That is one main thing they will always share and bond over. How they get to support and help, yet never quite be in the spotlight of their "celebrity" best friend.
The third rule is reciprocity. I feel as though this mostly comes in to play when they fight with each other, they both fight back. Also, when Hermione helps Ron out with homework sometimes she gets something in return. They do both support each other as they go through the hardships of fighting evil with HP as well.
All in all, it is not too surprising to see how they would end up together.