Its the natural order of things. But part of it is that stupid theory which people still follow, opposite attracts. Which is just dumb but some people swear by this. If you want a relationship to last longer, fucking pick someone that could be your best friend. Simple as that. Sometime its better to sightly lower your standard and pick up the girl youd be best buddy with, unless you want a quick lay. Sometimes you are lucky and you get both at once.