Valentine's Day always brings out a lot of talk about romance. Modern Western culture puts a lot of stock in love and romance -- mainly the endorphin-inducing sort, though the comfortable, long-term bonds of life partners get some recognition too.
Some other factors that people consider when choosing a life partner:
Money. This includes financial opportunities and other considerations that one's culture expects one to offer to one's life partner. Or the blood kin of one's life partner -- nepotism, to put it simply, which can turn out well or badly. Usually, this takes the form of business connections or an impressive investment portfolio, but it can include social networking and standard-of-living expectations. How much weight does your story's culture put on financial advantages in a marriage?
Children. The expectations that are put on children varies widely from culture to culture. What one's partner can offer one's children -- whether it's genetics, inheritance, or skills -- can have a big impact on the child's ability to fulfill those expectations. One intriguing, unfamiliar concept I've come across in my research is the (royal) child as the physical embodiment of a peace treaty (for example) between two nations. What if this physical symbol of unity were generated as needed, rather than within a life-long marriage? What symbolic importance could be put on the health and/or well-being of such a child?
Built in here is an assumption that there will be children. Which casts infertility in a different light. Historically, that's been blamed on the woman in the equation but what if a culture were more discerning?
Safety. This includes physical safety, traditionally the male's responsibility, and social safety which tends to lean on the female a bit more (depending on the culture, it might be mostly her job). What if these were switched?
Emotional validation. Humans need relationships without fear of rejection, they need to express themselves and be agreed with. Modern Western society has put one's partner as a primary source of that. In other times, other places, that role was taken by one's best friends or close relatives. Or, perhaps, lovers -- secret or otherwise. How would marriages be different if one expected one's most intimate emotional relationship to be with one's siblings? cousins?
Sex. Modern Western society does a lot of hyperventilating about the emotional power of sex -- and it's true, it can be powerful stuff. It can also be merely pleasant, kinda boring, and it can be downright awful. What if some other activity drew the kind of obsessive fascination (and simultaneous inhibition, if you're American) that our culture invests in sex?
Darker Radio (the recent "Free Music Friday" band Voyvoda in particular) for supplying me with a gloomy alt-rock soundtrack while I wrote this (LOL!) Yes, Darker Radio's page is in German and no, I do not sprecken the Deutsch. The bands they feature come from all over, though.