From a physical standpoint, Brock Lesnar is a rare human being. He has a rare combination of size, strength and general athletic ability. He also has shown a tendency to pack it in when things get difficult in his career choices.

Lesnar started training to be a pro wrestler in 2000, just after graduating college. By 2004 he had called it a career, having been one of the fastest rising stars in the history of the WWE. But the travel and the physical grind was too much for Brock. Something he described in his Death Clutch book:

My first year on the main roster in WWE was a blur. My second year was even worse. I was running into the grind. Same routine every day, day in and day out. The money was great, and I was buying a lot of nice things, but I had no time to enjoy any of it. That touring schedule just eats you up. I just kept thinking that there has to be a better way to make some real money.

Lesnar actually had upset many in the WWE by acting as though he was "above" the other members of the roster. Having just signed a new deal with the promotion, he decided that (even with his own personal jet to lessen travel issues) he wanted out. Maturity was seen as an issue for many as well, as seen in this post from the day of the news at the PW Torch:

Lesnar is said to be discontent backstage and was very upset that he had to fly all the way to South Africa "just to wrestle Hardcore Holly" instead of being used in a higher profile match on the tour. With his long term contract reportedly signed for 7 years, Lesnar doesn't feel he is being protected properly for the long term by the WWE. He is also upset that the current plan is for him to face the Undertaker as he feels he will likely "be fed" to the Undertaker, thus discounting his character. Lesnar isn't alone in this thinking as many have voiced their opinions against the return of the no-selling "Deadman" gimmick. He and McMahon were said to have had a very intense discussion last week resulting in his considering to take some time off after WM XX. As of this report, the WWE isn't making any post-WrestleMania plans involving Lesnar and have removed his name from all publicity material for upcoming shows.

The recent purchase of a $400,000 jet for his business travel remained the hot topic of conversation last week in both RAW & Smackdown locker rooms. While he did use his own money and receive the "OK" from WWE corporate, many still feel this was out of line as Ric Flair, Steve Austin, Undertaker, and others who have substantially more tenure and "weight" within the company have never done such. Individualism such as this caused immediate heat backstage for Lesnar who was also scrutinized for seemingly letting his push "go to his head" and other reported instances where his maturity was brought into question.

More about Lesnar's time in the WWE, NFL and UFC after the jump...

SBN coverage of UFC 141: Lesnar vs. Overeem

Many felt like his unwillingness to travel as a part of the "group" and not return the favor given to him where he had been "put over" by many more established wrestlers to build his character as a dominant force was an issue of Lesnar being very much a "kid" mentally.

Lesnar decided to chase a spot in the NFL after his WWE career ended. As a profile piece done by ESPN explained, Brock claimed that he was fully committed to a career in football:

"This is not some half-assed shot to see what I can do and try to make the NFL," Lesnar said. "For me, this is balls out, 100 percent."

While Lesnar's physicality impressed many in his attempt to make the Minnesota Vikings roster, those same old maturity issues popped up again. Lesnar was laying out quarterbacks with full speed hits, a big no-no in NFL training camps, and was involved in a handful of scuffles.

In the end it was decided that he wasn't ready for the NFL roster without a little more developing but he was invited to take a year in NFL Europe to develop. His agent told ESPN that Lesnar was ready to go through with the opportunity:

Lesnar wasn't available at camp to comment. His agent, Ed Hitchcock, said Lesnar wants to continue his development on the practice squad and in NFL Europe. "He's committed to this long-term," Hitchcock said. "He's realistic about what it takes to develop. There are players who've played this game all their lives who take a couple of years to develop [in the NFL]."

But Lesnar's attempt to make an NFL roster never moved past that invitation. Lesnar explained the situation in a Yahoo! article:

"You have to feel confident and I didn't," Lesnar said about his brief NFL experience. "I realized I was unprepared at the Vikings training camp. I was thinking to myself, 'What am I doing here?' Because of my lack of experience, I felt it wasn't for me."

What should have been an impressive feat (making the NFL Europe cut with relatively no experience is amazing) turned out to be just another time when Lesnar wasn't willing to make the sacrifices that came with the job.

In his UFC career, Brock was able to find success quickly once again. Remaining in Minnesota, he was able to work in the area he wanted and on a schedule that fit him better. While he dropped his first UFC bout when Frank Mir kneebarred him, he was able to get his career back on track and beat Heath Herring and then a Randy Couture who had been out of the cage for over a year to win the UFC heavyweight title.

The best win of Lesnar's career came at UFC 100 when he brutalized Frank Mir in their rematch to earn a second round TKO win. Brock took a shot at UFC sponsor Bud Light, promoted rival beer Coors Light and then said that he was going to go "climb on top of his wife" leading to having to apologize at the post fight presser.

Lesnar's battles with diverticulitis made his life extremely difficult over the past few years. No one can dispute that. But it's hard to not look at his decision to retire following his TKO loss to Alistair Overeem as making the decision to not gut through the hard times in his career yet again.

Lesnar has been able to make tremendous money in his life and that has allowed him to quit a promising wrestling career after only three years on the biggest stage, walk away from his NFL goals without accomplishing anything and now walk away from MMA after only roughly four years.

Brock has been a professional success, but I can't help but wonder if he has ever spent enough time trying to be the best to truly leave his mark on any pursuit.