Personally, I'm already happily employed and have just started grad school (masters). My motivation, aside from being more competitive in the job market (I don't want to stay in this job forever), is that I enjoy challenges. But I prefer to meet challenges at least somewhat prepared to take them on. I see grad school as an opportunity to study my field in more depth than I realistically have time or motivation for on the job or after work. I figure this will allow me sort of level up and be able to take on more challenging work in the future.