There's a simmering dislike, of course, a product of competing for English football's top prizes for more than 80 years on and off.
Batman hates The Riddler, but he hates The Joker more.
On Sunday, Old Trafford hosts the next clash of the not-so-mortal enemies. It's the first meeting since Sir Alex Ferguson's retirement and the start of a new rivalry between Arsene Wenger and David Moyes.
It was Wenger's appointment at Highbury in 1996 that reignited the feud between the two clubs that had briefly turned violent six years earlier.
Until then there had been a healthy competition. In the 1979 FA Cup final, United came back from 2-0 down against Arsenal with goals from Gordon McQueen and Sammy McIlroy in the last five minutes. But United's celebrations only lasted as long as it took Arsenal's Alan Sunderland to score a last-minute winner to cap the "Five-Minute Final."
The more recent tensions can be traced back to October 1990 and the Battle of Old Trafford. Nigel Winterburn's challenge on Denis Irwin sparked a mass brawl that resulted in United being deducted a point and Arsenal—who won the game 1-0—being deducted two, although it didn't stop George Graham's side winning the First Division title.
United did get some measure of revenge a month later when Lee Sharpe scored a hat-trick to inspire a 6-2 thumping at Highbury in the League Cup, a game that announced United as serious challengers to the established order of Liverpool and Arsenal.
But after Wenger arrived from France via Japan in October 1996, he quickly returned Arsenal to the top table where they found Ferguson's United already waiting.
They collided during Wenger's first full season in charge in 1997-98. Arsenal ate away at United's 11-point lead at the top of the Premier League to win their first title since 1991. Arsenal's 1-0 win at Old Trafford, thanks to Marc Overmars' late goal, was one of the pivotal games of the run-in.
It was the first of a series of explosive encounters that punctuated 17 years of Ferguson versus Wenger.
There was the FA Cup semi-final in 1999, Dennis Bergkamp's missed penalty, Roy Keane's red card and Ryan Giggs' hairy chest.
Dwight Yorke scored a 22-minute hat-trick in a 6-1 win in 2001, but little more than a year later, Sylvain Wiltord's goal won the title for Arsenal at Old Trafford.
Few United fans were left at the final whistle but those that were had to endure the painful sight of the Arsenal players celebrating on the pitch.
Ruud van Nistelrooy was at the centre of the next two controversial instalments. In 2003, he missed a last-minute penalty in a goalless draw at Old Trafford that provoked a wild response from a group of Arsenal players, led by Martin Keown.
The Dutchman got his revenge a year later with a penalty in United's 2-0 win which ended Arsenal's 49-game unbeaten run. The game become known as the Battle of the Buffet after an incident in the tunnel at the final whistle left Ferguson covered in pizza.
There was more incident in the reverse fixture at Highbury that season when Roy Keane and Patrick Vieira had to be separated in the tunnel by referee Graham Poll before the game had even started. It was triggered by Vieira confronting Gary Neville about his rough treatment of Jose Antonio Reyes.
Keane told Vieira he'd finished the dispute on the pitch before helping United win 4-2. But the French midfielder got his own back in the 2005 FA Cup final, scoring the winning penalty against United after a 0-0 draw at the Millennium Stadium.
Vieira left Arsenal for Juventus that summer while Keane moved to Celtic shortly after. That, combined with Arsenal falling away in the race for titles and trophies, soothed the rivalry between the clubs, and between Wenger and Ferguson.
But they have contested big games since including the 2009 Champions League semi-final, won by United 4-1 on aggregate thanks to John O'Shea's goal in the first leg and a supreme display of counter-attacking football in the second.
Wenger suffered one of his worst days as manager of Arsenal when his team were humiliated 8-2 at Old Trafford in August 2011 and the ill feeling between the two increased last summer when Robin van Persie swapped one red shirt for another.
Sunday marks the start of a new era in what has always been a tempestuous relationship.
Vieira, Keane, Neville, Reyes and now Ferguson have gone. But Wenger remains, and is in the process of building a team capable of ending their run of seven years without a trophy.
Inspired by Mesut Ozil's arrival and one-time United target Aaron Ramsey's form, Arsenal are five points clear at the top of the Premier League after 10 games.
Arsenal, it would seem, are back. And with it, their intense rivalry with United.