For decades, 65 was the traditional retirement age, but many of today's workers are setting their eyes on a finish line that's farther out, with some saying they plan to work as long as they're able to. There are several explanations for this, some good and some bad.

A recent Northwestern Mutual study listed the following five responses as the most common reasons Americans are choosing to remain in the workforce for longer.

1. I won't have saved enough to retire comfortably.

A lack of savings is a huge barrier to retirement and a problem facing an increasing number of adults today. Some of this comes down to rising living costs and life expectancies driving up retirement costs while pensions disappear and Social Security's future remains uncertain. Student loan debt and caring for aging parents is a factor in some cases. Many people are also guilty of underestimating their retirement expenses, and they might not realize it until they're nearing retirement.

A senior businesswoman talking on the phone at her desk

Image source: Getty Images.

Delaying retirement is a viable way to make up for a lack of retirement savings, but it's a risky plan because an illness, injury, or job loss could leave you out of work. Create a detailed retirement plan if you haven't already by multiplying your estimated annual expenses in retirement by the number of years of your retirement and adding 3% for inflation. Then, subtract any money you expect from other sources, like a pension, Social Security, or a 401(k) match, to figure out what you need to save on your own. Try to raise your monthly contribution accordingly, cutting back other expenses if you have to.

2. I enjoy my job and would like to continue.

Nearly three out of five of those surveyed reported that part of the reason they plan to work past 65 is because they enjoy what they do. This is a great reason to continue working, but don't neglect your retirement plan just because you think you'll want to keep working. As I mentioned above, an unexpected health emergency could force you to retire whether you want to or not. Or by the time you reach retirement age, you might be burnt out and want to retire after all.

If you're delaying retirement because you think you'll want to continue working, you should still plan as if you're going to retire at 65. That way, you'll have the option if you decide you want to, or you'll have an extra cushion to help you cover unexpected expenses when you do finally retire.

3. I do not feel like Social Security will take care of my needs.

The 56% of respondents who said they didn't believe Social Security would take care of them were right in a sense. Social Security was never intended to be a primary source of retirement income, though many retirees use it that way. It was only meant to cover about 40% of pre-retirement income for average workers, though the Social Security Administration gives no definition of average earnings. Your personal savings are supposed to cover the bulk of your expenses.

Many workers today fear that Social Security won't be around for them because the program's trust funds are projected to be depleted by 2035. But what most don't realize is that even if the government makes no changes to the program (which it likely will in time), Social Security would still be able to pay out 80% of its scheduled benefits until 2090.

There is a good chance Social Security might be worth less in the future, but it's not going away. You'll still be able to count on it for some help in retirement.

4. I am concerned about rising costs like healthcare.

This is another valid concern. Healthcare is already expensive, and the medical inflation rate often outstrips the general inflation rate, so it's likely to become even more costly in the future. Studies estimate that the average 65-year-old couple retiring today could spend anywhere from $285,000 to nearly $388,000, depending on how healthy they are. This doesn't include the portion of your healthcare expenses that Medicare covers, and it also doesn't include certain things, like hearing aids or long-term care, that Medicare doesn't cover at all.

You can use the above figures as a baseline when planning for healthcare costs in retirement. Build this extra amount into your retirement plan. If you need to keep working a little longer in order to save enough to cover your healthcare costs, it's probably worth it because a single major health expense could wipe out a good chunk of your savings. But there are always trade-offs. If your job causes you a lot of stress or encourages bad habits like sitting all day or eating fast food, working longer could actually make you more likely to develop health problems than if you were enjoying a healthy and active retirement.

5. I want additional disposable income.

Most people include their basic living expenses in their retirement plan, but it's difficult to predict what you might want to spend your money on in retirement. Using income from a job to cover discretionary purchases is a great way to ensure you don't completely derail your retirement budget. Consider looking for a part-time job if you're only using this money to cover discretionary items. That way, you'll still have some money coming in and the time to enjoy it.

Delaying retirement can be a beneficial source of additional income and a way to stay active and socialize with others. But just because you hope to work longer doesn't mean you'll be able to. Always do your best to plan as if you're going to retire anyway, just in case you have to.