According to the Pew Research Center, more people age 65 and over are working now than at any time since the turn of the century. And they’re spending more time on the job than in previous years, too. By 2020, an estimated one fourth of American workers will be 55 or older, up from 19 percent in 2010. But why? Are seniors working longer because they want to or because they have to?

The answer is — yes.

There are a number of reasons seniors are staying in the workforce, and although earning a paycheck is important it's only part of the story. Here's a look at some of the reasons why potential retirees are staying on the clock.

Because they need the money

Older hands count coins Retirement pensions and Social Security don't stretch as far as they used to. (Photo: Carol Heesen/Shutterstock)

Kato Cooks, 66, recently retired from his job as the chief compliance officer for the city of Palo Alto, California — not because he wanted to retire, but because the job contract simply ended. He is seeking a new job now, partially because he's bored and wants something productive to do with his days, but also because his retirement income from his public employees' pension and Social Security won't allow him to maintain his lifestyle.

Cooks is not alone in this realization. A few decades ago, retirees could count on combining Social Security with their company pension to live comfortably during retirement. But pension plans and Social Security payments are not what they used to be. And higher divorce rates means that many seniors are navigating the economics of retirement on their own. Combine these factors with increasing life spans, and it's easy to see why seniors are having trouble making ends meet.

Because they're living longer

Happy, healthy senior taking a selfie Thanks to advances in health care and nutrition, many seniors can look forward to a long retirement. (Photo: Rido/Shutterstock)

Americans are living longer than ever before, and this plays a big role in retirement. Older Americans are delaying retirement both because they feel still feel young enough continue working and because they realize they might live longer than they expected.

In 1985, the average life expectancy in the U.S. was 71.1 for men and 78.2 for women. Today, the average life expectancy for is 76.3 and 82.3, respectively, for the average man and woman. Those extra years equate to extra dollars needed in retirement.

A few decades ago, retirees could expect a retirement that might last 10 years, if they were lucky. But now retirement funds need to last for 10, 20 or even 30 years. That means seniors will either need to stretch those dollars further or wait longer to use them. For many seniors, this is a solid reason to stay on the job.

Another big health-related reason that retirees are staying in the workforce has to do with the massive cost of health care for seniors, even after they become Medicare-eligible. Patty Cathey, an investment adviser with Smart Retirement Plan in Denver, notes that many seniors work primarily to take advantage of their employer's health insurance benefits.

Because they want to stay connected

Senior at work Staying in the workforce allows seniors to stay connected with friends and coworkers. (Photo: StockLite/Shutterstock)

Author and public speaker Barry Maher always thought he would retire as soon as he could afford it. But once he reached retirement age, he realized just how much he truly loved what he did for a living.

For many seniors, the workplace is not just a place to make a paycheck; it's where their friends are and where they have an opportunity stay active mentally, physically and socially. In the latest annual Transamerica Retirement poll, 34 percent of retirees said they wanted to keep working because they enjoy their occupation or like the social and mental engagement of the workplace.

"For many, an avocation is a large sense of one’s identity," said Jennifer Myers, a financial adviser with SageVest Wealth Management. "For those individuals, retirement can translate into a loss of oneself. More importantly, it can translate into a loss of purpose. Staying active, whether employed or retired, is often key to happiness."

Let's face it, it's not easy to make new friends as you get older. Volunteering, social clubs and religious groups may help, but in many cases, seniors find themselves surrounded by younger people who are in different life stages or have different interests. In this sense, it's easier and more rewarding to stay on the job simply as a means to stay connected with others.

As Maher puts it, "I have a firm plan to retire the very moment my heart stops beating. Once my limbs start falling off, I may cut back a bit, but why would I ever retire?"

In addition to all of these benefits, working longer may also help you live longer, noted Dr. Noelle Nelson, author of "Happy Healthy…Dead: Why What You Think You Know About Aging Is Wrong and How To Get It Right." A recent University of Oregon study published in the Journal of Epidemiology and Community Health found that people who worked even just one year past age 65 had an 11 percent lower mortality risk than those who retired and were in similar health. The researchers concluded that "early retirement may be a risk factor for mortality and [that a] prolonged working life may provide survival benefits."