For some, they simply love what they do. For many more, there are just too many bills to pay.
In huge numbers, members of the baby boom generation — born from 1946 through 1964 — tell researchers that they don't plan to retire. In one recent AARP survey, nearly 70 percent of baby boomers reported they intend to work past the traditional retirement age of 65.
Those numbers have given rise to a fair amount of happy talk about how this generation is poised to reinvent retirement. Yet the retirement picture, like so much else for the nation's 78 million graying baby boomers, is complex.
On the one hand, baby boomers like to work: Despite a generational stereotype portraying them as free spirits who reject tradition, boomers in the prime of their working years have enthusiastically embraced the work ethic, often defining themselves by their careers.
Throwing on the career brakes at 65 simply sounds counterintuitive to many baby boomers. The working world has long embraced them, largely because they're better educated than the generations that came before or since, with almost 30 percent holding at least a bachelor's degree and another 30 percent having attended college.
But it's also true that with the death of traditional company pensions and more recently the biggest economic downturn since the Great Depression, the baby boom generation in many ways has no choice but to redefine what retirement means.
Many would leave the daily grind of jobs if they could. But continuing financial obligations to aging parents and grown children, as well as financial burdens left by the recession, have combined to make many boomers' retirement prospects more difficult.
"The reason that older participation in the workforce increased has nothing to do with the health and well-being of people that age," said social critic Susan Jacoby, author of "Never Say Die: The Myth and Marketing of the New Old Age."