Bill Gates's annual AMA (Ask Me Anything) has become one of the most popular question and answer sessions on Reddit. Users ask all kinds of questions, from inquiries about his favorite books to whether he still writes code, but in last year's session, one thought-provoking question rose to the top: Through it all, what makes you happy?

While Gates's answer was brief (he has to keep up with a barrage of questions, after all), he highlighted four main ingredients that have brought him happiness and keep him looking forward to his next 20 years.​ 

1. Stay true to your commitments.

When Gates was younger, he was committed to Microsoft's mission -- the idea of putting "a computer on every desk and in every home." That commitment involved countless hours and a fanatical approach to software, but over time, the vision became a reality.

As his original ambition was slowly realized, Gates needed to find a new commitment. Today, that's come in the form of the Bill and Melinda Gates Foundation, which is committed to eradicating infectious diseases such as malaria and polio and improving the living conditions of the world's poorest people -- which brings us to the second ingredient.

2. Give generously -- no matter your wealth.

As the second-richest person in the world, Gates can do or have anything he wants. That's why it's even more telling that the thing that drives him the most is his charitable giving. The Bill and Melinda Gates Foundation has been a tremendous philanthropic force over the years, and it's only increasing in momentum.

In 2006, Gates convinced his good friend Warren Buffett to donate $31 billion dollars of his fortune to the foundation, and the Giving Pledge emerged a few years later as an open invitation for other billionaires to do the same.

Since then, more than 200 of the world's wealthiest people have joined the Giving Pledge, dedicating the majority of their wealth to charitable efforts around the world.

While you might not have billions to set aside for philanthropy, everyone can contribute something to help other people.

All too often people imagine they'll give generously when they have more wealth to spare, but with that approach you risk coming to the end of your life without ever having truly contributed to an important cause.

On the other hand, start now with whatever time or money you can spare, and you'll be able to look back on a legacy of generosity that makes the world a better place.

3. Give your body the respect it deserves.

As an avid tennis player, Gates is aware of the link between exercise and happiness, and science supports his own anecdotal experience. The health benefits of exercise are difficult to overstate, from better weight control and reduced risk of heart disease to improvements to mental health and overall mood.

You get only one body, so it's crucial to treat it like a temple. According to one study of the cell cap telomeres -- that essential part of human cells that affect how our cells age -- physically active adults appeared to be nine years younger in terms of telomere length than their less active peers who were the same age. That's an astounding difference that highlights the fact that a sedentary lifestyle comes at a high cost.

4. Spend more time with your family.

Our culture has glorified the grind of the founder or entrepreneur to the point where people think 60- or 80-hour work weeks are a sign of dedication and success.

The reality is that this devotion is unhealthy and unsustainable. In fact, workplace stress is now the fifth-leading cause of death in the U.S., above even diabetes.

But besides the obvious health concern, putting in excessive hours at the office means spending less time somewhere else. We all have just 24 hours in every day.

Unfortunately for many workaholics, time with family is one of the first things to go when work hours start to ramp up. This decision almost inevitably leads to more conflict and more stress, further exacerbating health risks.

Protect the time you spend with your family by setting a few non-negotiable boundaries. Maybe you leave work on time every day and don't check emails or otherwise clock in again until you've put the kids to bed.

Put similar boundaries in place while you're at work to help you accomplish everything you need to in the time allotted.

Bringing it home

Gates's insights on happiness are incredibly illuminating, particularly because they all have one thing in common: accessibility. For all his billions, the things that have brought him happiness are surprisingly within reach.

Anyone can commit themselves to something and follow through with it, donate time or money to worthy causes, take steps to improve personal health, and spend more time with family or loved ones.

As we start a new year (and a new decade), choose these pursuits -- and choose your happiness as a result.