What motivates Tim Duncan to keep playing?

Sean Elliott once described Tim Duncan as "...an anomaly. He's a big man and a gym rat. That's an oxymoron. You have a lot of bigs who play because they're big. Guards play because they love it. Timmy has a guard's mentality. He has a guard's passion for the game in a big body. That's why he's been so good." So what motivates Tim Duncan - a man as accomplished as any in the post-MJ era - to continue to push through the physical and personal taxes of one season after another?

For a man as private as Duncan, it's unlikely that we will ever have a complete perspective on his life. What we do know is that the usual motivational factors for NBA players (money, haters, and legacy) just don't seem to apply. According to Pop Duncan and Kobe Bryant's motivation is "all in their heart, in their passion. [They're] highly professional, highly motivated [and] don't need any external motivation whatsoever. They're very special in that regard. They self motivate. They do what they need to do to stay on the court. They take care of themselves, and they love the game."

In a recent interview with Bleacher Report, RC Buford talked about how Duncan plays for something more than just his love for the game

"...I think he [Duncan] feels good. I think the way Pop manages his season, plays to him being at his best when it counts the most - and I think that's really important for him. So as long as he feels good, he gets great support from Draven and Sydney [Tim's kids]. So he actually gets to share this journey with the kids, and I know he enjoys that. While I'm sure there will be a time someday where he wants to stay home and spend as much time as he can, they share this journey with him now and I think they enjoy that and I think as a family they have a great time sharing that."

And like so many of Duncan's excellent qualities, the importance of family in his life stems from his childhood relationship with his mother.

Duncan's path to the NBA is well-documented because of how bizarre it was; having never played a minute of basketball until a hurricane devastated his dreams of becoming an Olympic swimmer. But less well known is the impact his mother had on his decision to stop swimming. In a San Antonio Express News article from 1997, Kevin O'Keeffe wrote

The following spring, the day before his 14th birthday in April 1990, came the real reason Duncan stopped swimming.

He lost his push.

Delysia Duncan died after a bout with breast cancer.

Understandably, her death had an incredible impact on him. He watched her suffer through the disease.

Delysia Duncan's passing expedited his growing up.

Cooperative, although hardly chatty and desiring media attention, Duncan made the subject of his mother's death off- limits through his first three years at Wake Forest. ... This season, though, Duncan opened up on that subject...

"His mom's death left a scar," said Deborah Best, chairman of the Wake Forest psychology department and Duncan's academic adviser.

"He talked to me about watching her die," Best told USA Today. "I think a lot of his independence today came out of losing his mom at such an early age."

Duncan said his mom gave him direction and was his biggest fan. He said he tuned out most people after his mom's death.

"No one replaces your mom," Duncan said.

It's ironic that Tim's biggest fan never saw him play a minute of basketball. If the 2013-14 season showed us anything, it's that his relationship with his own children may be just as close.

As historically brilliant as the Spurs' performance was in the 2014 playoffs, the season's most memorable moment occurred with the game clock off. Seldom does Duncan allow his emotions to show, which makes a moment like this all the more beautiful.

You don't need to know the intricate details of Duncan's life to understand how much that title meant to him, but the context of this hug is so rich (an ongoing messy divorce and the heartbreaking missed bunny in Game 7 the year before) that it seems straight out of a Hollywood script.

But Duncan's family encompasses more than just blood-relation. When Tim chose to remain in San Antonio, despite a tantalizing offer to trio up with Grant Hill and Tracy McGrady in Orlando, it was because of his relationship with Pop, the man Buford once described as Duncan's "soul mate."

Then there's his relationship with David Robinson:

And Stephen Jackson: "I'm humbled to be able to say that Tim Duncan is a good friend of mine."

And Tony Parker: "Timmy means a lot to me. I learned so much from him. From the first day I arrived in San Antonio he's always had my back. He's a very good friend, very close to me on the court and off the court."

And Manu:

Then there's what the Big Fundamental himself had to say about his NBA relationships:

Credit has to be given to David Robinson, Avery Johnson, Sean Elliott, Pop, etc. for building a healthy foundation, but Duncan has sustained it longer. The Spurs have been arguably the greatest sports franchise over the past 16 years because Tim was willing to sacrifice his ego for the benefit of his basketball family.

After winning the 2014 title, Duncan dubbed the championship run "very special. It was already a special day, but to do it at home, to do it with my kids and my family and my friends here made it so much more special. Just a great experience, and a great experience for them."

I doubt Timmy still plays for the money or his legacy. Maybe Pop is right and Tim keeps the engine revved because of his love of the game -- certainly it's part of it. But I keep coming back to the drive that he receives from being a part his two families.