For most of the boxing world, it’s apparent that a fight between Floyd Mayweather and Manny Pacquiao is long overdue. Not only have they proven to be the best fighters in the welterweight division, but they are, by far, the most popular boxers of their era and perhaps the best pound-for-pound boxers in well over a decade.

Since 2009, Pacquiao has pushed to make the bout happen, while Mayweather has declined to give fans what they want. The undefeated boxer has given reasons for choosing to fight other opponents, but it’s perplexing why he continues to put off what would be the biggest fight off all-time.

Is Mayweather afraid of Pacquiao?

It’s a prevailing thought among many fans, and even some experts, considering how much incentive Mayweather has to fight PacMan. Estimations have the mega-fight grossing close to $300 million, which would shatter the record of $150 million set by Mayweather vs. Canelo Alvarez in Sept. 2013.

Indeed, the amount of money on the table is staggering. No opponent could come close to generating the type of attention that Pacquiao brings, and Mayweather knows it. A Mayweather-Pacquiao bout could be the most talked about sporting event of 2015. 

Meanwhile, Mayweather's legacy may forever be tainted by his decision to not step into the ring with Pacquiao. For a boxer who cares deeply about his reputation, many may remember Mayweather for who he didn't fight rather than who he did.

With two fights left on his Showtime contract, Mayweather boasts a 47-0 career record. Some experts think that he doesn’t want to put his unbeaten mark in jeopardy.

Before his death in March 2012, famed boxing expert and historian Bert Sugar explained to International Business Times that boxing fans should not expect a mega-bout between Pacquiao and Mayweather, despite a highly lucrative payday available for both fighters.

“It will never happen,” said Sugar in Nov. 2011. “Mayweather is desperate to retire undefeated. He cares too much about his record to fight Pacquiao.”

Sugar described Pacquiao as “probably the greatest fighter I have seen in his style, in his weight class, since a fighter back in the 1930's, Henry Armstrong.”

Mayweather has provided a number of reasons (or excuses) for not fighting Pacquiao. Issues of drug testing and a split of the purse had come between the two sides, but Pacquiao agreed to Mayweather’s terms. His latest reason for not stepping in the ring with Pacquiao is his relationship with Top Rank CEO Bob Arum. Mayweather has refused to do business with the man who promoted him for a decade.

Pacquiao trainer Freddie Roach has been very vocal about his wish to see PacMan take on Mayweather. While he doesn’t think Mayweather fears a fight with Pacquiao, he agrees that Mayweather’s record has been the driving force in keeping the boxers out of the same ring.

"Floyd's main concern has become keeping that 0 [for zero losses] on his record,” Roach said, according to the Daily Mail, in April. “I don't see him taking any real risk of losing that before the end of next year, when he says he's retiring.”

"It's not that he's afraid of Manny. No fighters are scared of anybody in the world. It's just that he believes if he goes out undefeated he will be seen as better than greats like Sugar Ray Robinson, who had losses on their records.”

Mayweather beat Marcos Maidana on Sept. 13, but still doesn’t know who he’ll be facing in his next fight. Pacquiao’s convincing win over Chris Algieri gives him three consecutive victories, and he would be Mayweather’s toughest challenger, and certainly the most lucrative pay-per-view draw.

While talk of Mayweather vs. Pacquiao has increased, Amir Khan might be Mayweather’s most likely opponent. Mayweather's next fight will probably take place in May 2015.