Here are the quick answers for why this is so. People who feel sufficient personal power to fend for themselves tend to be Republican or Liberatarian, of or some similar ilk. This feeling of personal competence springs from inherent optimism. Optimistic people tend to be happy most of the time.
On the other hand, you have your leftists. They don’t have that feeling of being able to fend for themselves, and this causes their attraction towards increasing handouts from above, and increasing governmental control of their lives.
Since the government is inherently inefficient, it of course bungles the job of controlling leftist lives, and is unable to supply their needs. Sooooooo the leftists remain unhappy, and this increases with age and with accumulated disappointments.
Why are the old republican boys happier than the old republican girls? There are fewer boys left alive, at that stage of life, and more girls per boy, of course.
That was easy. :-)