The Cubs have won 101 games this year, the first time they've won at least 100 games since 1935. Their current winning percentage is .643, just shy of that year's .649.
Remarkably, their winning percentage could be higher, or maybe "should," depending on how you think of these things. Their run differential is now +252, 62 runs more than the Boston Red Sox, yet they've only won nine more games; they've scored 101 runs more than the Nationals, and have only won nine more games than Washington.
The idea is that a team's win-loss record—even though it's the only thing that actually matters—doesn't fully capture a team's performance, so there are other ways of calculating that. By Baseball Reference's calculations, they should currently be 105-52; by Fangraphs, 104-53; by Baseball Prospectus, they should have won between 105 and 112 games. So at the very high end of these estimates, a .716 winning percentage. That would be identical to the 116-win Mariners of 2001, the best team since the 111-43 (.721) Indians of 1954.
But the Cubs are arguably better than that Mariners team. The Cubs have (in theory) been unlucky, with their winning percentage underperforming their actual performance. The Mariners were the opposite; Baseball Prospectus has them as a true .679-.707 team.
Here's another way of looking at it. Baseball Reference calculates Wins Above Average—here's an explanation—for every team going back decades. I looked at WAA going back to 1962, when both leagues went to a 162-game season. By that measure, the 2016 Cubs are one of the best teams in modern history: the third best so far, with five games still to play.
(Yes, 1998 was a really top-heavy year; the Yankees won 114 games, the Braves 106, and the Astros 102. The National League-champion Padres, who lost to the Yankees in the World Series, won 98 games.)
The Cubs could probably squeeze out another Win Above Average, which would put them between the 2001 Mariners and the 1969 Orioles. So that's great; they deserve a place among the great teams of the modern era.
Here's the bad news. These teams had a mixed record in terms of the ultimate goal. Only the '98 Yankees, '07 Red Sox, '76 Reds, and '70 Orioles won the World Series. So if the Cubs lose, don't think of it as a curse or Cubs luck, but as just what happens to even the best laid plans in baseball.