i saw that fairly recently too. i think it goes back to the basic human need to feel secure/comfortable.
I've watched a ton of videos on ted.com about happiness, and I'll save everyone here the hours and just summarize
1. Devote yourself to something larger than yourself (this can take many forms, feeling part of a group (military, church, family, marriage, studies, work, helping others, etc))
2. Accept whatever comes your way (Einstein said "happiness isn't having what you want, it's wanting what you have") (Also a Dan Gilbert talk on ted.com that goes into this more)
3. Basic needs met (Maslow's hierarchy of needs basically, which goes back to the 60,000 a year)