Risk: far more seniors regret the risks they did not take than the ones they did.
Generations: many of our elders have troubled attitudes towards their own parents, but they seem extremely proud of what their own children have accomplished.
Divisions: the happiest people divided their history into chapters or phases, each with its own requirements and successes. The unhappy ones thought of themselves as corks bobbing up and down on the ocean of life, helpless to control their own fate.
Rebellion: for fifty years or more, our society has celebrated the outsider and the rebel. Brooks reports that the "most miserable" of his correspondents were always rebelling against the world and ended up not achieving much except sour memories. His advice is to work within groups or institutions to try to accomplish meaningful change (see Risk, above).
Boomers have been faulted for looking inward, for disrupting social conventions, and for rejecting many old-fashioned values. It's worth remembering that people we consider "elderly" today, the generation just a decade ahead of us, also lived through a time of opportunity and social change. It's nice to have their wisdom to guide us into the future.