Long-time former Googler here, author of go/l6talk (internal presentation on getting promoted to L6). While there I coached a lot of people on the L5-to-L6 promotion process. It seems a lot of Googlers get disillusioned about getting promoted above L5. Broadly speaking Google's culture is heavily focused on levels and promotions, which is too bad, since many L5's are doing just great and getting paid extremely well.

If it weren't for the fact that Google makes level information public within the company, I doubt there would be quite so much emphasis on getting a promotion. Most people want to get L6 for the recognition and the fact that it elevates you above other engineers in terms of status and influence. When I worked at Apple, levels were private, so you had to treat everyone the same -- a person's influence was not dictated by what level they had on their company profile (there is no company profile at Apple). In my opinion this approach was much better, and significantly lessened the internal competition for promotions.