Sounds like expectations of each other weren't being met. They were either not aware of these expectations, did not communicate the properly earlier on in the relationship or expectations have changed and they're just not willing to make compromises.
To me its really a state of mind culminating from a combination of things. A foreveralone could be super happy just having a girl around his place, doesnt even need coitus. Nagging may make him happy knowing that a person actually cares about him enough to nag at him. Rich person could be happy at a cheap homemade inexpensive gift, he has lots of money and can buy anything he wants. He cant buy time and effort from someone who cares.
I find the most rewarding and permanent way to keep myself happy is to change my perspective on things. Obviously its alot harder when working with 2 people in a relationship..... since their actions directly affect your state of mind. You'll constantly have to change your perspective to meet their needs and their perspective.
I'm a naturally happy person so lets flip it around and talk about what makes a person NOT happy. Expectations are probably the number 1 thing that kills my happiness. Its a viscious cycle, people are happy trying to obtain a goal, they reach it and they're happy for a bit but then they lose it because now their happiness/goal is done. Then they aim higher till eventually you start failing to meet expectations. Failing to meet expectations of yourself or others, or others failing to meet your expectations.
Check out these videos
http://www.ted.com/talks/tags/happinessTwo things that stuck with me from those videos were the points on:
-Low expectations will bring you more happiness
-Synthetic happiness is just as good as Actual happiness
I've also read this self helpish book on owning your own happiness. Pretty much goes over four points.
-Be honest about everything
-Dont make assumptions
-Dont take things personal
-Do your best
Its a little bit on the hokus pokus 'spiritual' side, but it teaches you to accept yourself, be confident in yourself and be a little selfish. I found the 'dont make assumptions' and 'dont take things personal' to be helpful.
In the end, I think people will be happiest when not thinking about happiness.