Well, one thing you have to look at is what Humanity's "hat" is in Mass Effect.
"Hat" is reference to the "Planet of Hats" trope on tvtropes.org. Basically, it's when an alien race is boiled down to a single defining character trait. All Klingons are honorable warriors, all Romulans are treacherous backstabbers, all Ferengi are greedy double-dealers. . . yeah, Star Trek pretty much lives and breathes that trope. Mass Effect tried to avert it, while at the same time getting some mileage out of it. All the races have stereotypes, but whether or not they live up to them is usually up to the individual, and some are outright lies (all Quarians are thieves.)
If humanity can be said to have a "hat" in Mass Effect, it's likely that we DO THINGS. Several people from various races comment on how humans are always out there, always doing stuff, always getting involved. We earned our Concil the fastest out of anyone. We've rapidly taken steps to integrate into the galactic community. Most importantly, we've done more to bring that community together in the past few years than the Council has done since it was created. Moreover, humanity are the ones with the plan to defeat the Reapers (literally, in terms of Crucible blueprints, and figuratively, in the form of getting stuff together to build thing and figuring out how to successfully deploy it.) Moreover, as other races are collapsing under the fear of Reaper attack (to say nothing of the actual attacks themselves), humans are the ones keeping their eye on the prize. Yeah, sure, we want to punt the Reapers off of Earth, but just taking Earth back is pointless unless we can proceed to hold it, and the only way to do that is by defeating the Reapers once and for all. The way I look at it, as the galactic situation grows steadily more desperate, all the aliens start falling in line with the human plan, because no one else has a better idea, or any idea at all.
As for the fleets going for Earth. . . that's where the Catalyst was. If the Citadel had stayed put, or been moved to Thessia, Palaven, Tuchanka, or West Armpit, you can be sure that's where the fleets would have gone, because that's where the battle needed to be fought. The battle at the end of the game had nothing to do with "taking back Earth." It had to do with taking back the Citadel and getting the Crucible fired up. For narrative reasons, to give the players an investment in the emotional arc of the story, the Citadel just happened to go to Earth, likely because, as has been pointed out, humans are responsible for the only real defeats the Reapers have had this cycle (and likely, for many, many cycles before.)
By the way. . . is anyone else noticing a fondness for C-named MacGuffins? Conduit, Crucible, Catalyst. . .