So my own little theory of how the Grumps came to be...

I know they already explained their "origins" in an episode, but I was thinking... They were pretty good friends before they started grumps, so what sparked the sudden idea to start a project like this together?

I think it's actually pretty simple; content! Both are skilled video creators, and love doing so... But they both also have a number of fans, (Arin more than Jon) and it seems that the more fans you have the less content you upload. So they - well, hatched is probably too dramatic for this but yeah - "hatched" an idea; create a Let's Play channel! They both love games, and are generally funny guys! The episodes wouldn't require as much effort as their own individual videos, but they could still enjoy creating content almost daily and uploading their large, high quality videos (as well as gaining help from each other - I mean, all you have to do is look at how Jon's newest video's quality has improved!) This also doubles as a bit of a reason for people to lay off them for not having constant content on their own channels... They have Grumps to tide you over, it's not a replacement for their shows just a way to each the pain!

TL;DR - Jon and Arin started GG because they wanted to have regular content that didn't need as much effort as their own shows.