At least that was up until the latest "Story Trailer" that was released during the VGAs and then un-edited by Sony. You can see it below. Either way it looks like there is a bit more on why Joel and Ellie have left the safety of the "Safe Zone" and ventured into hostile territory. I'll let the trailer explain more.

So it's because someone hired him to do it and no real explanation other than what we already know. What's the point here besides the new bit of game footage? I thought this was supposed to be a story trailer with something we didn't already know. I wasn't hoping for something that spoiled the story in the least bit, but something that explained a bit more on the overall reason for things. I guess on the plus side we have an official launch date of May 7^th 2013. I'm sure all will be explained then, unless they have sequels already planned and we are looking at a cliff hanger ending. Gods I hope not.