Starting one still looks rather random to me, you roll the dice until finally someone that you like likes you too, and that's most of what there's to it. Your attitude or effort can give you some small modifiers or penalties to the dice roll, but that's about it. It might sound childish but honestly, if I have to start a new relationship at some point, I'll be as lost in a sea of pure chaos as I was the last time.

On the other hand, keeping a relationship is pretty rational. Love the other person, care about her, don't be a dick, be emphatic, try to make her life as good as possible, and if you are not unusually unlucky you'll normally succeed.

Maybe it's also that as people that for one reason or other have a very hard time looking for relationships, we tend to value them more and make more effort to keep them than those who have it easier to get into one.