I think the reason people like it (myself included) is that it's likable. Not much deeper than that.

After dude leaves his job, there's essentially no story at all from there on. It's just watching a guy learn to be happy with his kid and friend. And of course the "father and son bond" and "man starts over his life, learns to be happy" stuff is unoriginal... but I'm fine with that. I love all manner of unoriginal subject matter, as long as I dig the execution. And I thought this was executed well.