Deciding when to retire is one of the biggest decisions you'll ever make, both financially and in your quality of life. But a recent survey found that many people are planning to work longer than previous generations -- and the reason why isn't what you probably think.

In the following video, Dan Caplinger, the Motley Fool's director of investment planning, looks at a study from Northwestern Mutual that compared what current retirees actually did versus how workers expect to retire. Dan notes that while the average retiree quit at age 59, most workers now expect to work until age 68. But the surprising thing is that fully 45% of those who expect to work that long are doing so because they want to, with financial considerations playing a secondary role. Dan points out that many people don't have that flexibility, with about 40% saying that they'll have to work to age 75 or later just to make ends meet. Nevertheless, Dan concludes that one goal of retirement planning should be to put yourself in a place where you can decide what to do based on what you want rather than what you need.