The last time this fight was all but a done deal, most blamed Mayweather for letting the deal fall through. Most thought that was because Mayweather was scared of Pacquiao and didn't want to take that sort of chance to taint his perfect record. I don't know if that was all of it, but I'm sure that played a role. Things seemed to have changed lately, though.
Pacquiao has fought Juan Manuel Marquez three times, and the record will show that Pacquiao is 2-0-1 in those fights. But, it seems all but the judges in those fights would probably say Pacquaio should have gone 1-2 at best in those fights, and quite possibly 0-2-1 or 0-3 depending on how you see the second fight. I haven't watched Marquez myself, but his style is usually compared to Mayweather, which obviously gives Pacquiao fits. This last real push by Mayweather started after Marquez was out and out robbed* in the last fight. I think Mayweather saw he could beat Pacquiao, even if a lot of sports-watching public** didn't. And, more importantly, I think Pacquaio (or at least Arum) thinks it, too.
*I wanted to see the fight, but the Danville BDubs doesn't do pay-per-views, so I couldn't. Still, this seems to be the consensus with everything I read immediately after the fight.
**Note that I said the sports watching public, not the hardcore boxing fans. I'm talking about people who watch a fair amount of ESPN and hear the Sportscenter and PTI take on everything.
For the curious, Mayweather and Marquez have also fought. Mayweather came away with an easy decision.
Will this fight ever happen? I really don't know. The two fighters are playing a chess game, it seems, and each side is only letting it happen when he's got the other in check. It seems like if they could just decide to hell with it and split a 50-50 purse and just fight in the ring instead of through the press, we'd all be much happier.