While every athlete learns the importance of sportsmanship at an early age, as they get older and become famous at the professional level, that notion can be tossed out the window.
Over the years, we've seen some famous feuds in sports that make fans think, "Do these guys really hate each other that much?"
If you're asking that question in the first place, then the answer is almost always "yes." And to prove how much they really despise each other, we're giving you 20 prime examples of athletes who truly can't stand one another.
As rivals throughout their tennis careers, Andre Agassi and Pete Sampras met on numerous occasions to decide the outcomes of major tournaments.
But as bitter as things were during their playing days, it didn't get real until they both retired and Agassi criticized Sampras in an autobiography.
Not to mention that whole tiff they had while playing for charity.
This picture might just be the happiest former teammates Tony Parker and Brent Barry have ever been together.
Sure, they may have won two NBA titles together, but that definitely doesn't mean they liked each other—especially after Parker allegedly hooked up with Barry's wife.
After divorcing former wife Eva Longoria in 2010 over "irreconcilable differences," reports came out that Barry's wife, Erin, was the woman Parker was seeing on the side and that Longoria found a bunch of texts from her on Parker's phone.
Titles or not, that's a good reason to hate another dude.
We're not saying Shannon Sharpe and Ray Buchanan necessarily hate each other. After all, this video came from Super Bowl media day, when one of the goals is to psych out the opponent.
But considering Sharpe said he wouldn't even pick Buchanan up if his truck broke down on the side of the road....well, I think that's more than just football talk.
Over the years, the relationship between teammates Jeff Gordon and Jimmie Johnson has been more than just rocky.
Gordon, who at one time was Johnson's mentor, took exception to being bypassed by J.J. on the circuit, which caused serious friction.
Some may claim that the two have put their differences aside, but when they stay away from each other, they seem to be at their happiest.
The Michael Jordan vs. Isiah Thomas spat has been going on for over two decades, and it probably isn't going to get resolved anytime soon.
After the Pistons threw everything but the kitchen sink at Jordan in the early '90s to eliminate the Bulls in the playoffs, the hate between these rivals extended away from just hard fouls and some trash-talking.
Jordan openly admitted to keeping Thomas off of the U.S. Olympic "Dream Team" of 1992—arguably the greatest team ever assembled—while Thomas has gone on record saying that LeBron James is a better athlete than M.J. ever was.
Peyton Manning is one of the best quarterbacks of all time. He holds a number of NFL passing records and is the only four-time league MVP. And he appears to be a nice guy.
But his former teammate, kicker Mike Vanderjagt, found out the hard way that Manning can be a little harsh.
Though it would be a fight between a kicker and quarterback—not known to be the biggest badass positions in football—it would be interesting to see what would unfold if they ever met in an alley.
When Jeff Kent made his way to the Bay Area in 1997, the MLB second baseman had already developed a reputation as a clubhouse cancer by getting into it with former teammates.
That anger came out in the worst way when he crossed paths with legendary slugger Barry Bonds, as the two just couldn't put their egos aside and get along.
Even with a World Series appearance together in 2002, their feud became public when they got into a shoving match in the dugout that same season, resulting in Kent saying, "I want off this team."
He signed with the Astros after the season, splitting up the former National League MVPs.
Shaquille O'Neal can say all he wants about how his hate towards former teammate Kobe Bryant was just something tactical for the media to talk about.
Though we've seen instances where the two have expressed respect for one another, what we've seen in the past has proven they aren't exactly the biggest fans of one another.
Considering Shaq openly said he "threatened to kill" Kobe in an autobiography, I'd say the two are more enemies than friends.
As two of the hardest-tackling enforcers in soccer, Roy Keane and Patrick Vieira gained the respect of their teammates and opponents during their days playing for rival clubs Manchester United and Arsenal.
Before the kickoff of a match between the two back in 2005, these two got into a heated exchange in the tunnel.
Listed as one of the most renowned pregame scuffles, these guys probably won't be enjoying bangers and mash with each other anytime soon.
Don't let Tiger Woods' and Phil Mickelson's smiles in this picture fool you—they aren't really that happy when in each other's company.
They're constantly in a battle to win tournaments as well as the adoration of fans, and they're presumed to have completely opposite personalities.
Mickelson is the family man, always with wife Amy and their kids, while Tiger remains very standoffish compared to other players on tour.
Even if you're not being the biggest NASCAR fan, the hatred between drivers Kyle Busch and Carl Edwards is one most casual fans even know about.
Having gotten into it several times throughout their racing careers, when the two are in contention during a race, there's no telling what might happen on the track.
One of the newer additions to the list, starting pitcher Zack Greinke and outfielder Carlos Quentin make this list after their confrontation on the diamond last month.
Getting plunked apparently triggered some bad memories for the former Chicago White Sox star, as Greinke hit him with a couple pitches during his time with the Kansas City Royals (both AL Central teams).
When a guy breaks the collarbone a guy who just signed one of the richest deals in baseball history, there's going to be some bad blood.
As arguably the two greatest fighters of their generation, Floyd Mayweather Jr. and Manny Pacquiao have a score to settle inside the ring. Unfortunately, they can't get on the same page to arrange the fight.
Despite all the promotional banter that goes on in the sport of boxing, the stuff that these two say about each other is personal, making everyone want to see a fight between them that much more.
As one of the biggest instigators in sports, it's not too surprising to see future Hall of Famer Kevin Garnett have some beefs with other guys.
On top of a well-documented feud with Tim Duncan in the past, Garnett has Knicks forward Carmelo Anthony to his hate list with the shenanigans he pulled earlier this season.
Alleged comments about his significant other irked Melo so much that he actually made his way to the Celts' bus after the game to try and settle things for good.
These two guys might be teammates on the Montreal Canadiens, but just because they wear the same colors doesn't mean P.K. Subban and Tomas Plekanec enjoy being around each other.
The two proved that as they got into a scuffle following practice in January 2012, giving new meaning to separation in the locker room.
Even though the two downplayed things, it's obvious there's some tension between them.
If Tony Parker's "sexting" relationship with the ex-wife of former teammate Brent Barry's was PG-13, then the drama surrounding former Chelsea teammates Wayne Bridge and John Terry is almost X-rated.
In 2010 before the World Cup, Terry allegedly had an affair with Bridge's then-girlfriend Vanessa Perroncel, with rumors that he knocked her up and then paid for an abortion.
Making things worse was that Bridge and Vanessa already had a child together.
Perroncel later denied the affair and other reports that she was sleeping with Bridge's other teammates on Chelsea, but that doesn't mean the two former mates wouldn't throw a few fisticuffs if given the chance.
After nailing Piazza in the head with a heater during the interleague series between the Mets and Yankees in the 2000 season—forcing Piazza to miss his start in the All-Star Game—these two were back for Round 2 come October.
After splitting the catcher's bat during Game 2 of that year's World Series, Clemens scooped it up and chucked it at Piazza as he made his way to first base.
Who knows what Clemens was thinking when he did this, but Piazza undoubtedly is not a fan of "The Rocket."
Normally, you look at a picture like this and think nothing of it.
It's just Netherlands international Rafael van der Vaart with two super attractive girls, right?
When you dig a little deeper, you find out that the blonde next to him is his since-separated wife Sylvie, while the brunette is her best friend, Sabia Boulahrouz, who also happens to be the wife of Rafael's teammate from club team Hamburg, Khalid Boulahrouz.
We have a strong feeling this won't end well for anyone involved.
This is one that's been going on for years, as the two quarterbacks used to see each other twice a year when Jay Cutler was still playing for the Denver Broncos.
Both Philip Rivers and Cutler are known to be a little brash and outspoken, so when they were seen jawing on the field years ago, it set off a trigger for both to dislike each other from then on.
Cutler later admitted he's "not a fan of they guy," in reference to Rivers, adding more fuel to the fire.
We mentioned the rivalry between Tiger Woods and Phil Mickelson earlier, but that's more borne of a competitive nature than anything.
Woods' hate for fellow golfer Sergio Garcia is very real, and the most recent display of it came this past weekend at The Players Championship.
For an in-depth breakdown of this rocky relationship over the years, check out this piece by fellow Bleacher Report featured columnist James McMahon.